tools/build/cross-build: Fix building libllvmminimal on Linux

There is a used member in glibc's posix_spawn_file_actions_t in
spawn.h, so we must temporarily undefine used when including it,
otherwise Support/Unix/Program.inc fails to build. This is based on
similar handling for __unused in other headers.

Fixes: 31ba4ce8898f ("Allow bootstrapping llvm-tblgen on macOS and Linux")
MFC after: 1 week