add new prctl command PR_SET_MM_MAP to sys/prctl.h

add new prctl command PR_SET_MM_MAP to sys/prctl.h

PR_SET_MM_MAP was introduced as a subcommand for PR_SET_MM in
linux v3.18 commit f606b77f1a9e362451aca8f81d8f36a3a112139e

the associated struct type is replicated in sys/prctl.h using
libc types.

example usage:

 struct prctl_mm_map *p;
 ...
 prctl(PR_SET_MM, PR_SET_MM_MAP, p, sizeof *p);

the kernel side supported struct size may be queried with
the PR_SET_MM_MAP_SIZE subcommand.