Declare a new `LINUX_STATIC` SDK and configure it.
Add options to set the build architectures for the `LINUX` and
`LINUX_STATIC` SDKs, similar to what we have for Darwin, because
we'll be cross-compiling.
Also add an option to point the build system at the sources for
the musl C library, which we're using for `LINUX_STATIC`.
rdar://123503470
