opcodes,gas: support for the ldtxa SPARC instructions.

This patch adds support for the LDTXA instructions, along with the
corresponding ASIs.  Tests for GAS are included.

opcodes/ChangeLog:

2016-07-12  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* sparc-opc.c (ldtxa): New macro.
	(sparc_opcodes): Use the macro defined above to add entries for
	the LDTXA instructions.
	(asi_table): Add the ASI_TWINX_* asis used in the LDTXA
	instruction.

gas/ChangeLog:

2016-07-12  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* testsuite/gas/sparc/ldtxa.s: New file.
	* testsuite/gas/sparc/ldtxa.d: Likewise.
	* testsuite/gas/sparc/sparc.exp: Execute the ldtxa test.