Fix compilation under newer GCC/Clang (#28130)

Linking coreclr with gcc 10 or clang 11 (both of which default to
-fno-common) fails with this error:

    [31%] Linking CXX shared library libdbgshim.so
    /usr/bin/ld: ../../pal/src/libcoreclrpal.a(dyn-info-list.c.o):src/pal/src/libunwind/src/mi/dyn-info-list.c:28:
    multiple definition of `_U_dyn_info_list'; ../../pal/src/libcoreclrpal.a(Linit.c.o):src/pal/src/libunwind/src/x86_64/Ginit.c:52:
    first defined here

This commit fixes that, by switching to -fcommon as the default.

Author: omajid

configurecompiler.cmake

@@ -478,6 +478,9 @@ if (CLR_CMAKE_PLATFORM_UNIX)
   #These seem to indicate real issues
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-invalid-offsetof>)
 
+  # Some newer compilers default to -fno-common; revert to -fcommon
+  add_compile_options(-fcommon)
+
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     # The -ferror-limit is helpful during the porting, it makes sure the compiler doesn't stop
     # after hitting just about 20 errors.