i386: Support .code64 directive only with 64-bit bfd

i386: Support .code64 directive only with 64-bit bfd

Without 64-bit bfd, we can't properly support .code64 directive in
32-bit mode.

	* config/tc-i386.c (md_pseudo_table): Add .code64 directive
	only if BFD64 is defined.
	* testsuite/gas/i386/code64-inval.l: New file.
	* gas/testsuite/gas/i386/code64-inval.s: Likewise.
	* gas/testsuite/gas/i386/code64.d: Likewise.
	* gas/testsuite/gas/i386/code64.s: Likewise.
	* testsuite/gas/i386/i386.exp: Run mixed-mode-reloc32,
	att-regs, intel-regs, intel-expr and string-ok tests only if
	assembler supports x86-64.  Run code64 and code64-inval.