arm: Add support for Armv8.7-A and Armv8.8-A

arm: Add support for Armv8.7-A and Armv8.8-A

This patch adds AArch32 support for -march=armv8.[78]-a.
The behaviour of the new options can be expressed using a
combination of existing feature flags and tables.

The cpu_arch_ver entries are technically redundant but
it seemed less surprising to include them anyway.

include/
	* opcode/arm.h (ARM_ARCH_V8_7A, ARM_ARCH_V8_8A): New macros.

gas/
	* doc/c-arm.texi: Add armv8.7-a and armv8.8-a.
	* config/tc-arm.c (armv87a_ext_table, armv88a_ext_table): New macros.
	(arm_archs): Add armv8.7-a and armv8.8-a.
	(cpu_arch_ver): Add ARM_ARCH_V8_7A and ARM_ARCH_V8_8A.
	* NEWS: Mention the above.
	* testsuite/gas/arm/attr-march-armv8_7-a.d: New test.
	* testsuite/gas/arm/attr-march-armv8_8-a.d: Likewise.
	* testsuite/gas/arm/bfloat16-armv8.7-a.d: Likewise.
	* testsuite/gas/arm/bfloat16-armv8.8-a.d: Likewise.
	* testsuite/gas/arm/i8mm-armv8.7-a.d: Likewise.
	* testsuite/gas/arm/i8mm-armv8.8-a.d: Likewise.