[ulib][musl] Rewrite setjmp/longjmp

The new implementation saves and restores the unsafe SP so that setjmp
calls in translation units built without -fsanitize=safe-stack will be
compatible with -fsanitize=safe-stack code elsewhere in their call
chains. It mangles the register values that usually hold stack or
code addresses stored in jmp_buf, so it's much harder to find such
addresses on the heap. It includes complete CFI for longjmp so that
every point unwinds correctly either to the caller of longjmp or to
the caller of setjmp in the context being restored.

Change-Id: I19fcb16a5e93a0219d3d840215f3695dccdb98b5

[musl] Move bits headers into public include

This makes the internal include paths much nicer (no .. paths).

Change-Id: Ida7a36c014a9408817010bdd1bf6285755143f25