[compiler-rt][cmake][mips] Correct the option handling for GCC 7

[atanasyan]

Summary

GCC 7 rejects the usage of --target=mipsel-linux-gnu, which earlier versions
accepted but warned on. Instead, detect the compiler being used and supply
--target=mipsel-linux-gnu for Clang only.

Additionally, -mips32r2 requires -mabi=32 for versions of GCC which target
mips64 by default.

This resolves PR37002.

Thanks to James Cowgill for reporting the issue!

[sdardis]

Ping.

[sdardis]

Ping.

[sdardis]

Ping.

[atanasyan]

Ping?

[atanasyan]

Initial problem has been fixed by rL343182.

cmake/base-config-ix.cmake

elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "powerpc")
       endif()
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "s390x")
       test_target_arch(s390x "" "")
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "mipsel|mips64el")
       # Gcc doesn't accept -m32/-m64 so we do the next best thing and use
       # -mips32r2/-mips64r2. We don't use -mips1/-mips3 because we want to match
       # clang's default CPU's. In the 64-bit case, we must also specify the ABI
       # since the default ABI differs between gcc and clang.
+      #
+      # We also have to tolerate differences between different versions of GCC
+      # and Clang, notably that --target=mips(el)-linux-gnu is accepted by both
+      # but required for Clang and gives a warning for GCC prior to 7. GCC 7
+      # rejects the option, causing a configuration issue leading cmake to
+      # believe GCC for mips, can't compile for mips.
+      set(CLANG_TARGET_FLAG "")
+      set(CLANG_TARGET_FLAG64 "")
+      if(CMAKE_C_COMPILER_ID MATCHES Clang)
+        set(CLANG_MIPS_TARGET_FLAG "--target=mipsel-linux-gnu")
+        set(CLANG_MIPS_TARGET_FLAG64 "--target=mips64el-linux-gnu")
+      endif()
       # FIXME: Ideally, we would build the N32 library too.
-      test_target_arch(mipsel "" "-mips32r2" "--target=mipsel-linux-gnu")
-      test_target_arch(mips64el "" "-mips64r2" "--target=mips64el-linux-gnu" "-mabi=64")
+      test_target_arch(mipsel "" "-mips32r2" "${CLANG_MIPS_TARGET_FLAG}" "-mabi=32")
+      test_target_arch(mips64el "" "-mips64r2" "${CLANG_MIPS_TARGET_FLAG64}" "-mabi=64")
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "mips")
-      test_target_arch(mips "" "-mips32r2" "--target=mips-linux-gnu")
-      test_target_arch(mips64 "" "-mips64r2" "--target=mips64-linux-gnu" "-mabi=64")
+      set(CLANG_TARGET_FLAG "")
+      set(CLANG_TARGET_FLAG64 "")
+      if(CMAKE_C_COMPILER_ID MATCHES Clang)
+        set(CLANG_MIPS_TARGET_FLAG "--target=mips-linux-gnu")
+        set(CLANG_MIPS_TARGET_FLAG64 "--target=mips64-linux-gnu")
+      endif()
+      test_target_arch(mips "" "-mips32r2" "${CLANG_MIPS_TARGET_FLAG}" "-mabi=32")
+      test_target_arch(mips64 "" "-mips64r2" "${CLANG_MIPS_TARGET_FLAG64}" "-mabi=64")
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "arm")
       if(WIN32)
         test_target_arch(arm "" "" "")
       else()
         test_target_arch(arm "" "-march=armv7-a" "-mfloat-abi=soft")
         test_target_arch(armhf "" "-march=armv7-a" "-mfloat-abi=hard")
         test_target_arch(armv6m "" "-march=armv6m" "-mfloat-abi=soft")
       endif()