Add musl git submodule

This adds a submodule of Musl git tag v0.9.12 as an initial step to
moving over to upstream Musl.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/string: Migrate to upstream musl

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/ctype: Migrate to upstream musl

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/locale: Migrate to upstream musl

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build.mk: Include libc/stdio directory

In preparation for switching source files to musl upstream, include
libc/stdio directory which has internal headers that musl needs.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/stdio: Migrate to upstream musl

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/internal: Remove math.h header file

The math.h header file is a broken duplicate of include/api/math.h which
forgets to pull <bits/alltypes.h> which defines "double_t" among others.

This is needed for migrating libc/math source files to upstream musl.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/math: Migrate to upstream musl

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>