[arm][1/3] Add -march=armv8.4-a option

[arm][1/3] Add -march=armv8.4-a option

This patch adds support for the Armv8.4-A architecture [1]
in the arm backend. This is done through the new
-march=armv8.4-a option.

With this patch armv8.4-a is recognised as an argument
and supports the extensions: simd, fp16, crypto, nocrypto,
nofp with the familiar meaning of these options.
Worth noting that there is no dotprod option like in
armv8.2-a and armv8.3-a because Dot Product support is
mandatory in Armv8.4-A when simd is available, so when using
+simd (of fp16 which enables +simd), the +dotprod is implied.

The various multilib selection makefile fragments are updated
too and the mutlilib.exp test gets a few armv8.4-a combination
tests.

Bootstrapped and tested on arm-none-linux-gnueabihf.

From-SVN: r256537