[arm] Add -march=armv8.3-a and dotprod multilib selection rules

[arm] Add -march=armv8.3-a and dotprod multilib selection rules

We don't have the t-aprofile, t-multilib and t-arm-elf mapping
rules for multilibs when using the variants of -march=armv8.3-a
and the dotproduct extension.
This patch adds them. -march=armv8.3-a behaves in the same
way as -march=armv8.2-a in this regard.

Bootstrapped and tested with the aprofile multilib list.
Checked that --print-multi-directory gives sensible results
with armv8.3-a options and extensions.
I've also added some armv8.3-a, fp16 and dotprod
combination tests to multilib.exp

	* config/arm/t-aprofile (MULTILIB_MATCHES): Add mapping rules for
	-march=armv8.3-a variants.
	* config/arm/t-multilib: Likewise.
	* config/arm/t-arm-elf: Likewise.  Handle dotprod extension.

	* gcc.target/arm/multilib.exp: Add fp16, dotprod and armv8.3-a
	combination tests.

From-SVN: r256345