Porting vlibc

How to port

To get the changes on tree please make a pull request, I can mantain and update them from there if needed.

The modifications to do to port vlibc are located in 3 folders, and are the following:

• source/crt: crt0, declared in a file for each OS/arch combination.
• headers-sysdeps: The headers representing the sysdeps along with OS-specific functions.
• vlibc/sysdeps: The actual V code for the sysdeps, one file per OS, and language features for separate architectures.

Headers necessary under headers-sysdeps

• host_mmap.h: PROT and MAP flags of the implementation.

Functions necessary under sysdeps

• sys_exit: POSIX-compatible _exit equivalent.
• sys_mmap: POSIX-compatible mmap equivalent.
• sys_munmap: POSIX-compatible munmap equivalent.