libc: arcmwdt: increase the number of preallocated locks

Increase the number of locks to match what the ARC MWDT libc requires
now. The library wants to have 2 locks per each available FILE entry,
and then some more. Also do not include an internal libc header, as
all that is needed from that header is a simple typedef.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>