configure.ac: fix HAVE_COMPATIBLE_GCC_AMD64_PLATFORM_AS on x32 targets
* configure.ac (gcry_cv_compiler_defines__x86_64__): New. (HAVE_COMPATIBLE_GCC_AMD64_PLATFORM_AS): Enable if __x86_64__ macro is defined by compiler and size of long is 4 (x32) or 8 (amd64).
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>