aarch64: Provide line info for unclosed sequences

We warn about MOVPRFX instructions that have no following
instruction.  This patch adds a line number to the message,
which is useful if the assembly code has multiple text sections.

The new code is unconditional since OBJ_ELF is always defined
for aarch64.

gas/
	* config/tc-aarch64.h (aarch64_segment_info_type): Add last_file
	and last_line.
	* config/tc-aarch64.c (now_instr_sequence): Delete.
	(force_automatic_sequence_close): Provide a line number when
	reporting unclosed sequences.
	(md_assemble): Record the location of the instruction in
	tc_segment_info.
	* testsuite/gas/aarch64/sve-movprfx_4.l: Add line number to error
	message.
	* testsuite/gas/aarch64/sve-movprfx_7.l: Likewise.
	* testsuite/gas/aarch64/sve-movprfx_8.l: Likewise.