[compiler-rt] Set CMAKE_TRY_COMPILE_TARGET_TYPE to STATIC_LIBRARY when building builtins standalone

When building builtins, the toolchain might not yet be at a stage
when linking a test application works yet, as builtins aren't
available. Therefore set CMAKE_TRY_COMPILE_TARGET_TYPE to STATIC_LIBRARY,
to avoid failing the compiler sanity check.

Setting CMAKE_TRY_COMPILE_TARGET_TYPE to STATIC_LIBRARY has the risk
of making checks for library availability succeed falsely (e.g.
indicating that libs would be available that really aren't, as the
tests don't do any linking), but the builtins library doesn't try to
link against any external libraries (and only produces static libraries
anyway), so it should be safe here.

This avoids having to set CMAKE_C_COMPILER_WORKS when bootstrapping a
cross toolchain, when building the builtins.

Differential Revision: https://reviews.llvm.org/D91334