configure: check $as first before using $gas as GNU as

llvm's integrated assembler supports the AArch64 asm on darwin since
August 2014. So check $as first before using gas-preprocessor.pl via
$gas. Makes the checks specific for that the architecture specific asm
needs. PPC Altivec and AArch64 needs on ':vararg' for macro arguments.
Arm needs in addition the '.altmacro' directive.