[PATCH][GAS][AArch64] Define BRBE system registers

[PATCH][GAS][AArch64] Define BRBE system registers

This patch introduces BRBE (Branch Record Buffer Extension) system
registers.

Note: as this is register only extension we do not want to hide these
registers behind -march flag going forward (they should be enabled by
default).

gas/ChangeLog:

2020-10-08  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* NEWS: Docs update.
	* testsuite/gas/aarch64/brbe-invalid.d: New test.
	* testsuite/gas/aarch64/brbe-invalid.l: New test.
	* testsuite/gas/aarch64/brbe-invalid.s: New test.
	* testsuite/gas/aarch64/brbe.d: New test.
	* testsuite/gas/aarch64/brbe.s: New test.

opcodes/ChangeLog:

2020-10-08  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>

	* aarch64-opc.c: Add BRBE system registers.