genfunctosyscallnrs: maps bionic functions to syscall numbers.

Bionic maps typical C functions like setresuid() to a syscall,
depending on the architecture used. This tool generates a .h
file that maps all bionic functions in SYSCALLS.txt to the
syscall number used on a particular architecture. It can then
be used to generate correct seccomp policy at runtime.

Example output in func_to_syscall_nrs.h:

Bug: 111434506
Test: manually inspect func_to_syscall_nrs.h
Change-Id: I8bc5c1cb17a2e7b5c534b2e0496411f2d419ad86