aarch64: Fix segfault on unicode symbols

This patch fixes a segfault which occurs when the AArch64 backend parses
a symbol operand that begins with a register name and ends with a
unicode byte (byte value > 127).

For example, the following input causes the crash:

x0é: udf x0é

gas/ChangeLog:

2020-07-02  Alex Coplan  <alex.coplan@arm.com>

	* config/tc-aarch64.c (reg_name_p): Fix cast so that we don't
	segfault on negative chars.
	* testsuite/gas/aarch64/reglike-label-unicode-segv.d: New test.
	* testsuite/gas/aarch64/reglike-label-unicode-segv.s: Input.