[Driver] Add --gcc-install-dir=

This option specifies a GCC installation directory such as
/usr/lib/gcc/x86_64-linux-gnu/12, /usr/lib/gcc/x86_64-gentoo-linux-musl/11.2.0 .

It is intended to replace --gcc-toolchain=, which specifies a directory where
`lib/gcc{,-cross}` can be found. When --gcc-toolchain= is specified, the
selected `lib/gcc/$triple/$version` installation uses complex logic and the
largest GCC version is picked. There is no way to specify another version in the
presence of multiple GCC versions.

D25661 added gcc-config detection for Gentoo: `ScanGentooConfigs`.
The implementation may be simplified by using --gcc-install-dir=.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D133329