libm does not exist on windows

Mingw provides a stub library for scripts that add it, but we're not
including it in our list of allowed libraries, since it's not necessary:

I'm probably going to be making it a default on darwin/linux shortly in
order to match the default set of libraries on Android (similar to how
-ldl / -lpthread / -lrt are defaults when available now). We can clean
up uses of that later.

Test: OUT_DIR=out python external/clang/ --no-multi-stage --no-pgo-clang --no-debug-clang
Change-Id: Ia3c897270bcdf689c4dec5a28fd63f3d87eda1c6
2 files changed