[sanitizer] Set default rtlib to libgcc

[rprichard]

Summary

The default --rtlib for Android is about to change from libgcc to
compiler-rt (D96404), which will require a more substantial change to
this script to build the builtins and libunwind. For now, keep using
libgcc.

[rprichard]

I also tested a version of this change where I instead added --rtlib=libgcc to the flags in configure_android. It was necessary to add --rtlib=libgcc both for compiling and linking, because at compile-time, Clang can use outline-atomics on AArch64. On other architectures, though, the --rtlib=libgcc has no effect at compile-time, and Clang printed a warning. It looks like this stage2 compiler is only used to target Android, though, so it seemed simpler to just override the default.

At some point, I think we'll want to update this script to build the builtins and libunwind before building the rest of compiler-rt.

zorg/buildbot/builders/sanitizers/buildbot_android_functions.sh

if  [[ ! -d platform-tools ]] ; then
     unzip ${FILE_NAME} > /dev/null
   fi
   export PATH=$ROOT/platform-tools/:$PATH
 }
 
 function build_stage2_android() {
   # Build self-hosted tree with fresh Clang and -Werror.
   local CMAKE_OPTIONS="${CMAKE_COMMON_OPTIONS} -DLLVM_ENABLE_WERROR=ON ${STAGE1_AS_COMPILER} -DCMAKE_C_FLAGS=-gmlt -DCMAKE_CXX_FLAGS=-gmlt"
-  CMAKE_OPTIONS="${CMAKE_OPTIONS} -DLLVM_ENABLE_PROJECTS='clang;lld' -DLLVM_USE_LINKER=lld"
+  CMAKE_OPTIONS="${CMAKE_OPTIONS} -DLLVM_ENABLE_PROJECTS='clang;lld' -DLLVM_USE_LINKER=lld -DCLANG_DEFAULT_RTLIB=libgcc"
 
   if ccache -s ; then
     CMAKE_OPTIONS="${CMAKE_OPTIONS} -DLLVM_CCACHE_BUILD=ON"
     rm_dirs llvm_build64
   fi
 
   echo @@@BUILD_STEP bootstrap clang@@@
   rm_dirs ${STAGE2_CLOBBER}