[Patch, ARM] Relax the restrictions on REG_SP under Thumb mode on ARMv8-A

	Backport from mainline
	2017-06-06  Jiong Wang  <jiong.wang@arm.com>
	* config/tc-arm.c (reject_bad_reg): Allow REG_SP on ARMv8-A.
	(parse_operands): Allow REG_SP for OP_oRRnpcsp and OP_RRnpcsp on
	ARMv8-A.
	(do_co_reg): Allow REG_SP for Rd on ARMv8-A.
	(do_t_add_sub): Likewise.
	(do_t_mov_cmp): Likewise.
	(do_t_tb): Likewise.
	* testsuite/gas/arm/ld-sp-warn.l: Delete the warning on REG_SP as Rt for
	ldrsb.
	* testsuite/gas/arm/sp-pc-validations-bad-t-v8a.d: New test.
	* testsuite/gas/arm/sp-pc-validations-bad-t-v8a.l: New test.
	* testsuite/gas/arm/sp-pc-validations-bad-t.d: Specifies -march=armv7-a.
	* testsuite/gas/arm/sp-pc-validations-bad-t.s: Remove ".arch armv7-a".
	* testsuite/gas/arm/sp-usage-thumb2-relax-on-v7.d: New test.
	* testsuite/gas/arm/sp-usage-thumb2-relax-on-v7.l: New test.
	* testsuite/gas/arm/sp-usage-thumb2-relax-on-v8.d: New test.
	* testsuite/gas/arm/sp-usage-thumb2-relax.s: New test.
	* testsuite/gas/arm/strex-bad-t.d: Specifies -march=armv7-a.