[compiler-rt][builtins] Fix CMake builtin target flag

clang-cl doesn't support -target <target>, instead it only supports
--target=<target> so building a RUNTIME configuration for clang-cl
ended up in never building builtins. Which in turn lead to clang-cl
not being able to find the runtime libraries because we depend
on the compiler_rt.builtins.lib being in the runtime dir for the
Driver to add it as a candidate.

I don't think this should have any downsides since most the code
these days are using --target=<target> instead of the old syntax.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D120308