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

[jsji]

Summary

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.

[Hahnfeld]

I don't feel comfortable reviewing this change, maybe @phosek can take a look.

[phosek]

LGTM

compiler-rt/cmake/base-config-ix.cmake

 # The CompilerRT build system requires CMake version 2.8.8 or higher in order
 # to use its support for building convenience "libraries" as a collection of
 # .o files. This is particularly useful in producing larger, more complex
 # runtime libraries.
 
 include(CheckIncludeFile)
 include(CheckCXXSourceCompiles)
+include(TestBigEndian)
 
 check_include_file(unwind.h HAVE_UNWIND_H)
 
 # Used by sanitizer_common and tests.
 check_include_file(rpc/xdr.h HAVE_RPC_XDR_H)
 if (NOT HAVE_RPC_XDR_H)
   set(HAVE_RPC_XDR_H 0)
 endif()
elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "i[2-6]86|x86|amd64")
         else()
           test_target_arch(x86_64 "" "")
         endif()
       endif()
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "powerpc")
       # Strip out -nodefaultlibs when calling TEST_BIG_ENDIAN. Configuration
       # will fail with this option when building with a sanitizer.
       cmake_push_check_state()
-      string(REPLACE "-nodefaultlibs" "" CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+      string(REPLACE "-nodefaultlibs" "" CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
       TEST_BIG_ENDIAN(HOST_IS_BIG_ENDIAN)
       cmake_pop_check_state()
 
       if(HOST_IS_BIG_ENDIAN)
         test_target_arch(powerpc64 "" "-m64")
       else()
         test_target_arch(powerpc64le "" "-m64")
       endif()