nolibc: support compilers which don't support -m32 and -m64 flags
nolibc: add uname program
nolibc: no longer depends on ld.so
Add mbr directory, with Syslinux MBRs
mbr: add README
mbr: add a Makefile which links the top-level Makefile with syslinux-mbr
nolibc: fix compilation

* Force entrypoint to be _start, whatever its address is.
* Include asm/unistd.h instead of sys/syscall.h to ease the use of almost-bare
  compilers.
nolibc: implement ARM syscall and entrypoint
nolibc: fix ARM build, which needs ld.so
nolibc: download required Linux kernel headers

This fixes building with ARM bare-metal compiler, using:

    make CC=arm-none-eabi-gcc
nolibc: no longer use ld.so when compiling with arm-none-eabi-gcc