mutex: add initial mutex support

We'll later use this to peg __sched, which will be our
first example subsection to test with linker tables.
Before we do the work to pget mutext to __sched we'll
add a bit more kernel interface support to make things
a bit more interesting and useful to test.

This a simple mutex implementation which just wraps around
the pthread pthread_mutex_t.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>