gas/x86: reduce / correct target checks for --64 command line option

gas/x86: reduce / correct target checks for --64 command line option

First, pei-x86-64 is meaningless for gas; it's a linker output target, not
one object files would use. Next, coff-x86-64 is meaningless for TE_PE
(and really coff-x86-64 isn't currently provided by any libbfd
configuration anyway). Then, of the three ones left exactly one is a
possible candidate for a given gas configuration. Checking others as well
would only lead to (possibly cryptic) errors later. And finally, even for
ELF we want to check for the one target which i386_target_format() would
also use. This last aspect then applies to --x32 handling as well (just
that there it's benign right now, as only one target exists starting
"elf32-x86-64".