Add `-no-toolchain-stdlib-rpath` flag: the negative version of
`-toolchain-stdlib-rpath`.
Make `-no-toolchain-stdlib-rpath` be the default: use `/usr/lib/swift` as
default RPATH on Darwin platforms instead of toolchain standard library.
Adapted from https://github.com/apple/swift/pull/27206.
tensorflow branch requires the opposite default (use toolchain standard
library as RPATH) because some stdlib modules like TensorFlow do not exist in
`/usr/lib/swift`.
