Add --with-extra-multilib-test option

NOTE: This is toolchain developer facing feature.

This allow user to add extra testing multi-lib arch, it's useful when
develop and/or testing new extensions.

Usage:

--with-extra-multilib-test="arch-abi[;arch-abi]"

e.g.

linux enable enabled multilib, so default will build with follwoing
configuration:

lib32/ilp32;@march=rv32imac@mabi=ilp32
lib32/ilp32d;@march=rv32imafdc@mabi=ilp32d
lib64/lp64;@march=rv64imac@mabi=lp64
lib64/lp64d;@march=rv64imafdc@mabi=lp64d

But you want to testing more on vector stuffs like rv32gcv and rv64gcv,
then you can configure with `--with-extra-multilib-test="rv32gcv-ilp32d;rv64gcv;lp64d"`

Then the testing will run rv32imac-ilp32 rv32imafdc-ilp32d
rv64imac-lp64 rv64imafdc-lp64d and rv32gcv-ilp32d;rv64gcv;lp64d!

NOTE: Extra multilib test settings still require existing
multilib has support those extra settings, e.g. you can't add
rv32imafc_zbb-ilp32f on above example since no compatible multilib has
provided.