Generate a warning in the ARM assembler if a PC-relative thumb load instruction is...

Generate a warning in the ARM assembler if a PC-relative thumb load instruction is detected in a section with insufficient alignment.

	* config/tc-arm.c (md_apply_fix): Warn if a PC-relative load is
	detected in a section which does not have at least 4 byte
	alignment.
	* testsuite/gas/arm/armv8-ar-it-bad.s: Add alignment directive.
	* testsuite/gas/arm/ldr-t.s: Likewise.
	* testsuite/gas/arm/sp-pc-usage-t.s: Likewise.
	* testsuite/gas/arm/sp-pc-usage-t.d: Finish test at end of
	disassembly, ignoring any NOPs that may have been inserted because
	of section alignment.
	* testsuite/gas/arm/ldr-t.d: Likewise.