Fix the generation of alignment frags in code sections for AArch64.

	PR gas/20364
	* config/tc-aarch64.c (s_ltorg): Change the mapping state after
	aligning the frag.
	(aarch64_init): Treat rs_align frags in code sections as
	containing code, not data.
	* testsuite/gas/aarch64/pr20364.s: New test.
	* testsuite/gas/aarch64/pr20364.d: New test driver.

Cherry-pick of 7ea12e5c3ad54da440c08f32da09534e63e515ca

Change-Id: I6bb37f33419cbf54e52c8d54ef0d1c18fdc85878
4 files changed