[Driver] Check normalized triples for multiarch runtime path

Previously we only used target triple as provided which matches the
GCC behavior, but it also means that all clients have to be consistent
in their spelling of target triples since e.g. x86_64-linux-gnu and
x86_64-unknown-linux-gnu will result in Clang driver looking at two
different paths when searching for runtime libraries.

Unfortunatelly, as it turned out many clients aren't consistent in
their spelling of target triples, e.g. many Linux distributions use
the shorter spelling but config.guess and rustc insist on using the
normalized variant which is causing issues. To avoid having to ship
multiple copies of runtimes for different triple spelling or rely on
symlinks which are not portable, we should also check the normalized
triple when constructing paths for multiarch runtimes.

Differential Revision: https://reviews.llvm.org/D50547

llvm-svn: 340471

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