aarch64: Define CSRE system registers

aarch64: Define CSRE system registers

This patch introduces CSRE (Call Stack Recorder 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).

CSRE feature adds CSR PDEC (Decrements Call stack pointer by the size
of a Call stack record) instruction. This instruction will be added in
a following, separate patch. This change only adds CSRE system
registers.

gas/ChangeLog:

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

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

opcodes/ChangeLog:

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

	* aarch64-opc.c: New CSRE system registers defined.