[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[musl] Introduce MTX_INIT extension for statically initializing mtx_t

This is gated behind the _ALL_SOURCE feature test macro. This patch also
places our thrd_create_with_name behind the same, as it should have been
when created.

Change-Id: I34b545d9cd07b1beaae2e406e87510e4856ff65b

[musl] Implement C11 mutexes in terms of runtime, not pthreads

Change-Id: I9d613790f86a309041d78c0500a08071bf890cc0

[magenta] Initial commit