Now it outputs libc.S which can be assembled with zig, and the small
differences per-architecture are handled with preprocessor directives.
There is also now a set of blacklisted symbols which contains
compiler-rt.
tools/gen_stubs.zig now cuts out the middle man and operates directly on
the libc.so ELF file. it outputs accurate .size directives for objects.
std.elf gains an STV enum.
