[Driver] Fix finding multilib gcc install on Gentoo (with gcc-config)

Fix the gcc-config code to support multilib gcc installs properly. This
solves two problems: -mx32 using the 64-bit gcc directory (due to matching
installation triple), and -m32 not respecting gcc-config at all (due to
mismatched installation triple).

In order to fix the former issue, split the multilib scan out of
Generic_GCC::GCCInstallationDetector::ScanLibDirForGCCTriple() (the code
is otherwise unchanged), and call it for each installation found via
gcc-config.

In order to fix the latter issue, split the gcc-config processing out of
Generic_GCC::GCCInstallationDetector::init() and repeat it for all
triples, including extra and biarch triples. The only change
in the gcc-config code itself is adding the call to multilib scan.

Convert the gentoo_linux_gcc_multi_version_tree test input to multilib
x86_64+32+x32 install, and add appropriate tests to linux-header-search
and linux-ld.

Differential Revision: https://reviews.llvm.org/D26887

llvm-svn: 289436