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
This change enables the `--gc-sections` linker flag for WebAssembly
targets by default. This flag has been disabled because it did not
respect `llvm.used` and it caused stripping `swift5` metadata sections
even though they were used through encapsulation symbols (a.k.a `__start`/`__stop`).
The issue has been fixed in the LLVM side (ba3c1f9ce3)
by adding new segment flags to the WebAssembly object file format.
