MdePkg: Check if compiler has __has_builtin before trying to use it

When building AArch64 code, cpp gets run with the `-undef` flag which on
Fedora 40 with gcc version 14.1.1 20240607 (Red Hat 14.1.1-5) causes
__has_builtin to be undefined. When running the check for
__builtin_unreachable in Base.h it causes an error
"missing binary operator before token "("".

Check that we have __has_builtin before trying to use it.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
1 file changed