* config/tc-aarch64.c (set_other_error): New function.
(parse_sys_reg): Add new parameter 'sys_reg' and if non-NULL set
the variable to which it points with 'o'.
(parse_operands): Update; check for write to read-only system
registers or read from write-only ones.
gas/testsuite/
* gas/aarch64/diagnostic.s: Add tests.
* gas/aarch64/diagnostic.l: Update.
* gas/aarch64/tracereg-illegal.d: New file.
* gas/aarch64/tracereg-illegal.l: Ditto.
* gas/aarch64/tracereg-illegal.s: Ditto.
* gas/aarch64/tracereg.d: Ditto.
* gas/aarch64/tracereg.s: Ditto.
include/opcode
* aarch64.h (aarch64_sys_reg_readonly_p): New declaration.
(aarch64_sys_reg_writeonly_p): Ditto.
opcodes/
* aarch64-opc.c (CPENT): New define.
(F_READONLY, F_WRITEONLY): Likewise.
(aarch64_sys_regs): Add trace unit registers.
(aarch64_sys_reg_readonly_p): New function.
(aarch64_sys_reg_writeonly_p): Ditto.
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