aarch64: Don't assert on long sysreg names

aarch64: Don't assert on long sysreg names

This patch fixes an assertion failure on long system register operands
in the AArch64 backend. See the new testcase for an input which
reproduces the issue.

gas/ChangeLog:

	* config/tc-aarch64.c (parse_sys_reg): Don't assert when parsing
	a long system register.
	(parse_sys_ins_reg): Likewise.
	(sysreg_hash_insert): New.
	(md_begin): Use sysreg_hash_insert() to ensure all system
	registers are no longer than the maximum length at startup.
	* testsuite/gas/aarch64/invalid-sysreg-assert.d: New test.
	* testsuite/gas/aarch64/invalid-sysreg-assert.l: Error output.
	* testsuite/gas/aarch64/invalid-sysreg-assert.s: Input.

include/ChangeLog:

	* opcode/aarch64.h (AARCH64_MAX_SYSREG_NAME_LEN): New.