[Driver] Use arch type to find compiler-rt libraries (on Linux)

Use llvm::Triple::getArchTypeName() when looking for compiler-rt
libraries, rather than the exact arch string from the triple. This is
more correct as it matches the values used when building compiler-rt
(builtin-config-ix.cmake) which are the subset of the values allowed
in triples.

For example, this fixes an issue when the compiler set for
i686-pc-linux-gnu triple would not find an i386 compiler-rt library,
while this is the exact arch that is detected by compiler-rt. The same
applies to any other i?86 variant allowed by LLVM.

This also makes the special case for MSVC unnecessary, since now i386
will be used reliably for all 32-bit x86 variants.

Differential Revision: https://reviews.llvm.org/D26796

llvm-svn: 311836