Support "ah", "bh", "ch", "dh" on amd64.

Support "ah", "bh", "ch", "dh" on amd64.

gdb/

2010-04-01  H.J. Lu  <hongjiu.lu@intel.com>

	* amd64-tdep.c (amd64_byte_names): Add "ah", "bh", "ch", "dh".
	(AMD64_NUM_LOWER_BYTE_REGS): New.
	(amd64_pseudo_register_read): Handle "ah", "bh", "ch", "dh".
	(amd64_pseudo_register_write): Likewise.
	(amd64_init_abi): Set num_byte_regs to 20.

gdb/testsuite/

2010-04-01  H.J. Lu  <hongjiu.lu@intel.com>

	* gdb.arch/amd64-byte.exp: Check "ah", "bh", "ch", "dh".