AArch64: Close sequences at the end of sections

AArch64: Close sequences at the end of sections

Any open sequence at the end of a section or assembly is considered an error.
This patch adds a check at the end to ensure that all sequences have been closed
and if not reports a warning.

During disassembly it's not possible to detect this condition in the back-end so
the warning is only emitted from the assembler for now.

gas/

	* config/tc-aarch64.c (force_automatic_sequence_close,
	aarch64_frob_section): New.
	* config/tc-aarch64.h (tc_frob_section, aarch64_frob_section): New.