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 <[email protected]>

MdePkg/Include/Base.h

@@ -59,7 +59,7 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 /// up to the compiler to remove any code past that point.
 ///
 #define UNREACHABLE()  __builtin_unreachable ()
-  #elif defined (__has_feature)
+  #elif defined (__has_builtin) && defined (__has_feature)
     #if __has_builtin (__builtin_unreachable)
 ///
 /// Signal compilers and analyzers that this call is not reachable.  It is