[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] Remove the extended locale functions

The C library provides locale aware variants of several ctype.h and
string.h functions, but all these functions completely ignore the
locale setting and simply call the non-locale variant. Rather than
pretending that our C library supports locale, we should omit these
functions for now and if needed, actually implement them in the
future with the proper locale support.

Change-Id: Ibc7eeda464232340c2dc78d63331b6b02b1b7341

[musl] Fix GNU-ish declaration of basename in string.h

This was originally declared as

char* basename();

and incorrectly modified to

char* basename(void);

when -Werror=strict-prototypes was introduced. musl declares it as such
presumably because glibc declares it complicatedly as either trafficking
in char* or in const char* depending on various things. Given that the
only implementation we provide in fact modifies the string, just declare
char* and call it a day.

Change-Id: Ia663f348099f01c238a9ca3690cb49624a1b12c3

[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