Separate construction of bi-arch path suffix from construction of
multi-library path suffix.

The code calculates MIPS toolchain specific multi-lib path suffixes like
mips16/soft-float/el is moved to the separate function
findMultiLibSuffix(). This function called during GCC installation
detection and result is stored for the future using.

The patch reviewed by Rafael Espindola.
http://llvm-reviews.chandlerc.com/D1738

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191612 91177308-0d34-0410-b5e6-96231b3b80d8