[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

[wchar_t] Fix the definition of WCHAR_MIN and add tests

This adds a definition of WCHAR_MIN that works in C and C++, and under
GCC and Clang. The two compilers provide different predefined macros
for libc's need to provide this value.

This patch also duplicates a lot of the existing stdint tests to make
sure things work in both C and C++, and that they work when only
<wchar.h> is included.

Change-Id: I90bc0aed3dd921015adbca2d8ec6cd43ce3c737a

[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 userspace int types and format specifiers

This also fixes two bugs in the kernel libc equivalents.

In addition, remove a number of unneeded inclusions of <sys/types.h>
throughout system, and fix some format specifiers. Also add a test of
these things (which found the bugs mentioned above).

Change-Id: I4051394276340b00673510494eb9b8942dc55561

[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