configure.ac: Add check for aarch64 assembler -mabi support.

2013-12-11  Kugan Vivekanandarajah  <kuganv@linaro.org>

	* configure.ac: Add check for aarch64 assembler -mabi support.
	* configure: Regenerate.
	* config.in: Regenerate.
	* config/aarch64/aarch64-elf.h (ASM_MABI_SPEC): New define.
	(ASM_SPEC): Update to substitute -mabi with ASM_MABI_SPEC.
	* config/aarch64/aarch64.h (aarch64_override_options): Issue error
	if assembler does not support -mabi and option ilp32 is selected.
	* doc/install.texi: Added note that building gcc 4.9 and after
	with pre 2.24 binutils will not support -mabi=ilp32.

From-SVN: r205891