add new syscall numbers for bpf and kexec_file_load

these syscalls are new in linux v3.18, bpf is present on all
supported archs except sh, kexec_file_load is only allocted for
x86_64 and x32 yet.

bpf was added in linux commit 99c55f7d47c0dc6fc64729f37bf435abf43f4c60

kexec_file_load syscall number was allocated in commit
f0895685c7fd8c938c91a9d8a6f7c11f22df58d2
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.