Provide bindings for libc

[densh]

For sake of having nice interop with C we need to have access to all the same base libraries. The full list of header files is given below:

• [x] assert.h (not useable without C-style macros.)
• [x] complex.h
• [x] ctype.h
• [x] errno.h
• [x] fenv.h
• [x] float.h
• [x] inttypes.h
• [ ] iso646.h
• [ ] limits.h
• [x] locale.h
• [x] math.h
• [ ] setjmp.h
• [x] signal.h
• [ ] stdalign.h
• [ ] stdarg.h
• [x] stdatomic.h
• [ ] stdbool.h
• [ ] stddef.h
• [ ] stdint.h
• [x] stdio.h
• [x] stdlib.h
• [ ] stdnoreturn.h
• [x] string.h
• [x] tgmath.h
• [ ] threads.h
• [ ] time.h
• [ ] uchar.h
• [ ] wchar.h
• [ ] wctype.h

Each header would map to a single extern object in scala.scalanative.libc package. Some of the headers might not map directly or need some additional compiler magic, we need to discover those cases sooner rather than later.