Avoid name clash when using with RP Runtimes

RP Runtimes provide some symbols with the same `External_Name` as
`rp2040_hal`. So, to avoid "multiple definition" linker error
we preppend a prefix when setting `use_startup=false`.

Author: reznikmm

src/drivers/rp-rom-floating_point.ads

@@ -9,23 +9,35 @@ is
    procedure Initialize
      with Export,
           Convention    => C,
-          External_Name => "__gnat_initialize_bootrom";
+          External_Name => External_Name_Prefix & "__gnat_initialize_bootrom";
    --  __gnat_initialize_bootrom must be called before any of the __aeabi
    --  functions, which means it needs to be called before elaboration.
    --  crt0.S takes care of this, unless Use_Startup=false.
 
    --  Run-time ABI for the Arm® Architecture
    --  https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst
    function fadd (A, B : Float) return Float
-      with Export, Convention => C, External_Name => "__aeabi_fadd";
+   with Export,
+        Convention => C,
+        External_Name => External_Name_Prefix & "__aeabi_fadd";
+
    function fsub (A, B : Float) return Float
-      with Export, Convention => C, External_Name => "__aeabi_fsub";
+   with Export,
+        Convention => C,
+        External_Name => External_Name_Prefix & "__aeabi_fsub";
+
    function frsub (A, B : Float) return Float
       with Export, Convention => C, External_Name => "__aeabi_frsub";
+
    function fmul (A, B : Float) return Float
-      with Export, Convention => C, External_Name => "__aeabi_fmul";
+   with Export,
+        Convention => C,
+        External_Name => External_Name_Prefix & "__aeabi_fmul";
+
    function fdiv (A, B : Float) return Float
-      with Export, Convention => C, External_Name => "__aeabi_fdiv";
+   with Export,
+        Convention => C,
+        External_Name => External_Name_Prefix & "__aeabi_fdiv";
 
    --  __aeabi_cfcmpeq
    --  __aeabi_cfrcmple

src/drivers/rp-rom.ads

@@ -5,6 +5,7 @@
 --
 with Interfaces.C; use Interfaces.C;
 with Interfaces;   use Interfaces;
+with Rp2040_Hal_Config;
 with System;
 
 package RP.ROM
@@ -47,12 +48,18 @@ is
        Code  : Table_Code)
        return System.Address;
 
+   External_Name_Prefix : constant String :=
+     (if Rp2040_Hal_Config.Use_Startup then "" else "RP_");
+   --  This prefix is prepend to external names to avoid name clash with
+   --  symbols in Ada runtimes (light_tasking_rp2040, etc) when setting
+   --  use_startup = false.
+
    function ROM_Func_Lookup
       (Code : Table_Code)
       return System.Address
    with Export,
         Convention    => C,
-        External_Name => "__gnat_rom_func_lookup";
+        External_Name => External_Name_Prefix & "__gnat_rom_func_lookup";
 
    function ROM_Data_Lookup
       (Code : Table_Code)