[aarch64] Make common aarch64 options target-dependent

[aarch64] Make common aarch64 options target-dependent

This patch replaces the "Common" attribute in some aarch64 options with "Target"
to make them target-dependent. This doesn't affect their behaviour or how they
are documented with "--help", but does mean that they follow the option
specification that target-specific options are given the "Target" attribute.

Tested by ensuring the changed options are still accepted as before and that
they aren't invokable from an invalid target. Bootstrapped and regression tested
on aarch64-none-elf with ... regressions.

2018-08-16  Sam Tebbs  <sam.tebbs@arm.com>

	* config/aarch64/aarch64.opt (mlow-precision-recip-sqrt)
	(mlow-precision-sqrt, mlow-precision-div, mverbose-cost-dump): Replace
	"Common" with "Target".

From-SVN: r263578