[ulib][musl] Use the compiler's <stddef.h>

The compiler provides one that has all we need, so just use it.
Clean up headers defining NULL to use a shared definition, which
in turn is outsourced to the compiler's <stddef.h>.

This uncovered some C++ test code that was conflating uintptr_t
with pointers and comparing it to NULL, which worked out OK before
when musl's sloppy C++ NULL definition was being used but is caught
by the proper definition from the compiler's <stddef.h>.

TO-441

Change-Id: I0b055d2c6e4e2f94b517f3aad782dfc1ebe7e767

[musl] Use #pragma once in musl's public headers

Like the prior commit, this only replaces existing ifdef guards.

Change-Id: I39be129111db9f129aef72a53f986ce6a49376ac

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit