Use host's executable suffix for clang when cross-compiling compiler-rt

When cross-compiling compiler-rt as part of LLVM e. g. for Linux on
a Windows host and using the just-built clang as cross-compiler, we set
the -DBUILTINS_CMAKE_ARGS="-DCMAKE_SYSTEM_NAME=Linux" flag in top-level
cmake invocation, which causes CMAKE_EXECUTABLE_SUFFIX to be an empty
string in the nested cmake invocation for building builtins.

But the compiler for compiling test cases is meant to be run on host,
therefore it may have the '.exe' suffix.

Handle this by asking cmake about the host system.

Patch by Sergej Jaskiewicz <jaskiewiczs@icloud.com>

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D67401

llvm-svn: 371754