x86: Don't display --32/--64/--x32 without BFD64

x86: Don't display --32/--64/--x32 without BFD64

For 32-bit x86 assembler, --64 and --x32 are unsupported if BFD64 is
undefined.  Even if BFD64 is defined, --64 and --x32 still may not be
supported if x86-64 support isn't compiled in:

[hjl@gnu-hsw-1 gas]$ ./as-new --64 -o x.o x.s
Assembler messages:
Fatal error: no compiled in support for x86_64
[hjl@gnu-hsw-1 gas]$ ./as-new --x32 -o x.o x.s
Assembler messages:
Fatal error: no compiled in support for 32bit x86_64
[hjl@gnu-hsw-1 gas]$

This patch removes --32/--64/--x32 from md_show_usage if BFD64 is
undefined and runs code64-inval only if BFD64 is undefined.

	* config/tc-i386.c (md_show_usage): Don't display --32/--64/--x32
	if BFD64 is undefined.
	* testsuite/gas/i386/i386.exp (gas_bfd64_check): New.
	Run code64-inval if gas_bfd64_check fails.