[compiler-rt][CMake] Fix PowerPC runtime build

When build in runtime bulid mode with LLVM_ENABLE_RUNTIMES,
the base-config-ix.cmake will complain about two errors.

One is empty string in replace, the other one is unknown `TEST_BIG_ENDIAN ` command.

This patch fix it so that we can test runtime build.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D80040