lib: os: add sys_mutex data type

For systems without userspace enabled, these work the same
as a k_mutex.

For systems with userspace, the sys_mutex may exist in user
memory. It is still tracked as a kernel object, but has an
underlying k_mutex that is looked up in the kernel object
table.

Future enhancements will optimize sys_mutex to not require
syscalls for uncontended sys_mutexes, using atomic ops
instead.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>