kernel: export k_sched_lock and k_sched_unlock.

Oversight. These functions are used extensively in the kernel guts, but
are also supposed to be an API.

k_sched_lock used to be implemented as a static inline. However, until
the header files are cleaned-up, and everything, including applications
get access to the kernel internal data structures, it must be
implemented as a function. To reduce the cost to the internals of the
kernel, the new internal _sched_lock() contains the same implemetation,
but is inlined.

Change-Id: If2f61d7714f87d81ddbeed69fedd111b8ce01376
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>