x86: bring "gas --help" output for --32 etc in sync with reality

x86: bring "gas --help" output for --32 etc in sync with reality

The testsuite uses the output to determine whether BFD64 is in effect.

--x32 is supported for ELF only; don't advertise it for PE/COFF. --64 is
also supported for Mach-O; advertise it. Adjust the testsuite's BFD64
check accordingly.

Also replace "code" by "object", since it's the object format that the
options primarily control. It's also _initial_ code bitness, but this
can be changed by directives.