Despite them being ignored by the CPU, gas issues segment override

Despite them being ignored by the CPU, gas issues segment override
prefixes for other than FS/GS in 64-bit mode. If doing so at all, it
should clearly do this correctly. Determining the default segment,
however, requires to take into consideration RegRex (so far, RSP, RBP,
R12, and R13 were all treated equally here).

gas/
2012-08-07  Jan Beulich <jbeulich@suse.com>

	* config/tc-i386-intel.c (build_modrm_byte): Split determining
	default segment from figuring out encoding. Honor RegRex for
	the former.

gas/testsuite/
2012-08-07  Jan Beulich <jbeulich@suse.com>

	* gas/i386/x86-64-segovr.{s,l}: New.
	* gas/i386/i386.exp: Run new test.