[arm] Don't pass -mfpu=auto through to the assembler.

The assembler doesn't understand -mfpu=auto.  The easiest way to handle this
is to surpress this value from being passed through, while still passing
through legacy values.

	* config/arm/elf.h (ASM_SPEC): Only pass -mfpu through to the
	assembler when it is not -mfpu=auto.

From-SVN: r249281