[Mips] Allocate NaClTargetInfo for MIPSEL NaCl

A patch to AllocateTarget function to recognize llvm::Triple::NaCl for
MIPSEL and return NaClTargetInfo. Additional test has been added to check
if the expected macros get defined.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191124 91177308-0d34-0410-b5e6-96231b3b80d8