serpent-amd64: do not use GAS macros

* cipher/serpent-avx2-amd64.S: Remove use of GAS macros.
* cipher/serpent-sse2-amd64.S: Ditto.
* configure.ac [HAVE_COMPATIBLE_GCC_AMD64_PLATFORM_AS]: Do not check
for GAS macros.

This way we have better portability; for example, when compiling with clang
on x86-64, the assembly implementations are now enabled and working.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>