[compiler-rt][cmake][powerpc] Remove TEST_BIG_ENDIAN from base-config-ix.cmake

[daltenty]

Summary

It's actually not safe to call TEST_BIG_ENDIAN here, since we may be
running from the builtins build (i.e builtins-config-ix) context where
TEST_COMPILE_ONLY is set since without builtins already built we may
fail to link, and TEST_BIG_ENDIAN internally performs tests which may
fail to link without builtins.

Fortunately powerpc is the only target that uses this information here and
we actually already know the whether we are targeting the LE variant due
to earlier macro checks, so we can simply this to remove our reliance on
TEST_BIG_ENDIAN.

[hubert.reinterpretcast]

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("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "i[2-6]86|x86|amd64")
         endif()
       else()
         if (CMAKE_SIZEOF_VOID_P EQUAL 4)
           test_target_arch(i386 "" "")
         else()
           test_target_arch(x86_64 "" "")
         endif()
       endif()
+    elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "powerpc64le")
+      test_target_arch(powerpc64le "" "-m64")
     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}")
-      TEST_BIG_ENDIAN(HOST_IS_BIG_ENDIAN)
-      cmake_pop_check_state()
-
-      if(HOST_IS_BIG_ENDIAN)
-        if(CMAKE_SYSTEM_NAME MATCHES "AIX")
-          test_target_arch(powerpc "" "-m32")
-        endif()
-          test_target_arch(powerpc64 "" "-m64")
-      else()
-        test_target_arch(powerpc64le "" "-m64")
-      endif()
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "s390x")
       test_target_arch(s390x "" "")
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "sparc")
       test_target_arch(sparc "" "-m32")
       test_target_arch(sparcv9 "" "-m64")
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "mipsel|mips64el")
       # Gcc doesn't accept -m32/-m64 so we do the next best thing and use
       # -mips32r2/-mips64r2. We don't use -mips1/-mips3 because we want to match