Hello,

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.

Testing:
 * New test which fails before and passes after the patch.
 * Testsuite run on x64 -> aarch64-none-elf build with no regressions.

OK for master?

Thanks,
Alex

---

2020-07-21  Alex Coplan  <alex.coplan@arm.com>

gas/ChangeLog:

	* config/tc-aarch64.c (parse_sys_reg): Reject system registers longer
	than 31 bytes instead instead of asserting.
	* 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.