Add gentoo gmp

[Lucretia]

Added the package name for Gentoo because:

$ alr with libadalang_tools
Requested changes:                                                                                                  

   ✓ libadalang_tools ^25.0.0 (add)
                                                                                
Changes to dependency solution:

   New solution is incomplete.
   +        adasat           25.0.0 (new,indirect)
   +📦      gnat             16.0.0 (new,gnat_external,indirect,executable in path)
   +        gnatcoll         25.0.0 (new,indirect)
   +        gnatcoll_gmp     25.0.0 (new,indirect)
   +        gnatcoll_iconv   25.0.0 (new,indirect)
   +        langkit_support  25.0.0 (new,indirect)
   +        libadalang       25.0.0 (new,indirect)
   +        libadalang_tools 25.0.0 (new)
   +        libgpr           25.0.0 (new,indirect)
   +        libgpr2          25.0.0 (new,indirect)
   +        prettier_ada     25.0.0 (new,indirect)
   +        templates_parser 25.0.0 (new,indirect)
   +        vss              25.0.0 (new,indirect)
   +        xmlada           25.0.0 (new,indirect)
   Missing:
   +❗      libgmp           *      (new,indirect,missing)

Do you want to proceed?
[Y] Yes  [N] No  (default is No) y
ⓘ Deploying adasat=25.0.0...
##O=#    #                                                                                                                
ⓘ Deploying vss=25.0.0...                                                         
      -=O=- #     #       #       #                                                                                       
ⓘ Found 1 nested crate in /home/laguest/.local/share/alire/releases/vss_25.0.0_1ddbb26c:
   tools/json_schema/json_schema=1.0.0: JSON Schema to Ada converter
ⓘ Deploying xmlada=25.0.0...
#=#=- #    #                                                                                                              
ⓘ Deploying libgpr=25.0.0...                                                      
 -#O=#    #     #                                                                                                         
ⓘ Deploying templates_parser=25.0.0...                                            
##O=#    #                                                                                                                
ⓘ Deploying gnatcoll=25.0.0...                                                              
   #-=O#-   #     #                                                                                                       
ⓘ Deploying gnatcoll_gmp=25.0.0...                                                  
##O=#    #                                                                                                                
ⓘ Deploying gnatcoll_iconv=25.0.0...                                                    
##O#-  #                                                                                                                  
ⓘ Deploying prettier_ada=25.0.0...                                                        
#=#=- #    #                                                                                                              
ⓘ Deploying langkit_support=25.0.0...                                                   
         -=O=-#      #       #       #                                                                                    
ⓘ Deploying libgpr2=25.0.0...                                                              
################################################################################################################### 100.0%
ⓘ Deploying libadalang=25.0.0...                                                   
################################################################################################################### 100.0%
ⓘ Deploying libadalang_tools=25.0.0...                                                
 -=#=- #    #     #                                                                                                       
warn: The following external dependencies are unavailable within Alire:                     
warn:    libgmp*
warn: Index 'my_private_alire_index' version (1.1.0) is older than the newest supported by alr (1.4.0)
warn: You can disable this warning with settings key 'warning.old_index'
ⓘ If you experience any problems loading this index, you may need to reset the community index with 'alr index --reset-community'. Note that this operation will delete any local changes to the community index.
warn: They should be made available in the environment by the user.
warn: The following external dependencies are unavailable within Alire:
warn:    libgmp*
warn: They should be made available in the environment by the user.

[Lucretia]

Also need alire-project/alire#2006 to be applied.

[github-actions]

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