Handle invalid prefixes for rdrand and rdseed

Handle invalid prefixes for rdrand and rdseed

This patch puts rdrand and rdseed in prefix_table so that invalid
prefixes for rdrand and rdseed are handled properly.

gas/testsuite/

	PR binutils/17898
	* gas/i386/prefix.s: Add rdrand/rdseed prefix tests.
	* gas/i386/prefix.d: Updated.

opcodes/

	PR binutils/17898
	* i386-dis.c (PREFIX_0FC7_REG_6): Renamed to ...
	(PREFIX_MOD_0_0FC7_REG_6): This.
	(PREFIX_MOD_3_0FC7_REG_6): New.
	(PREFIX_MOD_3_0FC7_REG_7): Likewise.
	(prefix_table): Replace PREFIX_0FC7_REG_6 with
	PREFIX_MOD_0_0FC7_REG_6.  Add PREFIX_MOD_3_0FC7_REG_6 and
	PREFIX_MOD_3_0FC7_REG_7.
	(mod_table): Replace PREFIX_0FC7_REG_6 with
	PREFIX_MOD_0_0FC7_REG_6.  Use PREFIX_MOD_3_0FC7_REG_6 and
	PREFIX_MOD_3_0FC7_REG_7.