[Driver][Mips] Construct dynamic linker path by string concatination.
That reduces a number of `if` operators and prevent a combinatorics explosion
if/when more dynamic linker path variants added.

No functional changes.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@214712 91177308-0d34-0410-b5e6-96231b3b80d8