[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