Compiler-rt: Fix MIPS CMake variables set

Move the code which sets CMake variables for MIPS to
compiler-rt/cmake/base-config-ix.cmake from compiler-rt/CMakeLists.txt.

compiler-rt/CMakeLists.txt includes compiler-rt/cmake/base-config-ix.cmake
very early. If the variables are set in compiler/CMakeLists.txt,
compiler-rt/cmake/base-config-ix.cmake cannot use them.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D157900