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
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.