aarch64: Fix bogus type punning in parse_barrier() [PR26699]

This patch fixes a bogus use of type punning in parse_barrier() which
was causing an assembly failure on big endian LP64 hosts when attempting
to assemble "isb sy" for AArch64.

The type of the entries in aarch64_barrier_opt_hsh is
aarch64_name_value_pair. We were incorrectly casting this to the
locally-defined asm_barrier_opt which has a wider type (on LP64) for the
second member. This happened to work on little-endian hosts but fails on
LP64 big endian.

The fix is to use the correct type in parse_barrier(). This makes the
locally-defined asm_barrier_opt redundant, so remove it.

gas/ChangeLog:

	* config/tc-aarch64.c (asm_barrier_opt): Delete.
	(parse_barrier): Fix bogus type punning.
	* testsuite/gas/aarch64/system.d: Update disassembly.
	* testsuite/gas/aarch64/system.s: Add isb sy test.