Implement Read/Write constraints on system registers on AArch64

This patch adds constraints for read and write only system registers with the
msr and mrs instructions.  The code will treat having both flags set and none
set as the same.  These flags add constraints that must be matched up. e.g. a
system register with a READ only flag set, can only be used with mrs.  If The
constraint fails a warning is emitted.

Examples of the warnings generated:

test.s: Assembler messages:
test.s:5: Warning: specified register cannot be written to at operand 1 -- `msr dbgdtrrx_el0,x3'
test.s:7: Warning: specified register cannot be read from at operand 2 -- `mrs x3,dbgdtrtx_el0'
test.s:8: Warning: specified register cannot be written to at operand 1 -- `msr midr_el1,x3'

and disassembly notes:

0000000000000000 <main>:
   0:	d5130503 	msr	dbgdtrtx_el0, x3
   4:	d5130503 	msr	dbgdtrtx_el0, x3
   8:	d5330503 	mrs	x3, dbgdtrrx_el0
   c:	d5330503 	mrs	x3, dbgdtrrx_el0
  10:	d5180003 	msr	midr_el1, x3	; note: writing to a read-only register.

Note that because dbgdtrrx_el0 and dbgdtrtx_el0 have the same encoding, during
disassembly the constraints are use to disambiguate between the two.  An exact
constraint match is always prefered over partial ones if available.

As always the warnings can be suppressed with -w and also be made errors using
warnings as errors.

binutils/

	PR binutils/21446
	* doc/binutils.texi (-M): Document AArch64 options.

gas/

	PR binutils/21446
	* testsuite/gas/aarch64/illegal-sysreg-2.s: Fix pmbidr_el1 test.
	* testsuite/gas/aarch64/illegal-sysreg-2.l: Likewise.
	* testsuite/gas/aarch64/illegal-sysreg-2.d: Likewise.
	* testsuite/gas/aarch64/sysreg-diagnostic.s: New.
	* testsuite/gas/aarch64/sysreg-diagnostic.l: New.
	* testsuite/gas/aarch64/sysreg-diagnostic.d: New.

include/

	PR binutils/21446
	* opcode/aarch64.h (F_SYS_READ, F_SYS_WRITE): New.

opcodes/

	PR binutils/21446
	* aarch64-asm.c (opintl.h): Include.
	(aarch64_ins_sysreg): Enforce read/write constraints.
	* aarch64-dis.c (aarch64_ext_sysreg): Likewise.
	* aarch64-opc.h (F_DEPRECATED, F_ARCHEXT, F_HASXT): Moved here.
	(F_REG_READ, F_REG_WRITE): New.
	* aarch64-opc.c (aarch64_print_operand): Generate notes for
	AARCH64_OPND_SYSREG.
	(F_DEPRECATED, F_ARCHEXT, F_HASXT): Move to aarch64-opc.h.
	(aarch64_sys_regs): Add constraints to currentel, midr_el1, ctr_el0,
	mpidr_el1, revidr_el1, aidr_el1, dczid_el0, id_dfr0_el1, id_pfr0_el1,
	id_pfr1_el1, id_afr0_el1, id_mmfr0_el1, id_mmfr1_el1, id_mmfr2_el1,
	id_mmfr3_el1, id_mmfr4_el1, id_isar0_el1, id_isar1_el1, id_isar2_el1,
	id_isar3_el1, id_isar4_el1, id_isar5_el1, mvfr0_el1, mvfr1_el1,
	mvfr2_el1, ccsidr_el1, id_aa64pfr0_el1, id_aa64pfr1_el1,
	id_aa64dfr0_el1, id_aa64dfr1_el1, id_aa64isar0_el1, id_aa64isar1_el1,
	id_aa64mmfr0_el1, id_aa64mmfr1_el1, id_aa64mmfr2_el1, id_aa64afr0_el1,
	id_aa64afr0_el1, id_aa64afr1_el1, id_aa64zfr0_el1, clidr_el1,
	csselr_el1, vsesr_el2, erridr_el1, erxfr_el1, rvbar_el1, rvbar_el2,
	rvbar_el3, isr_el1, tpidrro_el0, cntfrq_el0, cntpct_el0, cntvct_el0,
	mdccsr_el0, dbgdtrrx_el0, dbgdtrtx_el0, osdtrrx_el1, osdtrtx_el1,
	mdrar_el1, oslar_el1, oslsr_el1, dbgauthstatus_el1, pmbidr_el1,
	pmsidr_el1, pmswinc_el0, pmceid0_el0, pmceid1_el0.
	* aarch64-tbl.h (aarch64_opcode_table): Add constraints to
	msr (F_SYS_WRITE), mrs (F_SYS_READ).

include/opcode/aarch64.h

@@ -765,7 +765,11 @@ extern aarch64_opcode aarch64_opcode_table[];
 #define F_LSE_SZ (1 << 27)
 /* Require an exact qualifier match, even for NIL qualifiers.  */
 #define F_STRICT (1ULL << 28)
-/* Next bit is 29.  */
+/* This system instruction is used to read system registers.  */
+#define F_SYS_READ (1ULL << 29)
+/* This system instruction is used to write system registers.  */
+#define F_SYS_WRITE (1ULL << 30)
+/* Next bit is 31.  */
 
 static inline bfd_boolean
 alias_opcode_p (const aarch64_opcode *opcode)