tools/nolibc: add a makefile to install headers

This provides a target "headers_standalone" which installs the nolibc's
arch-specific headers with "arch.h" taken from the current arch (or a
concatenation of both i386 and x86_64 for arch=x86), then installs
kernel headers. This creates a convenient sysroot which is directly
usable by a bare-metal compiler to create any executable.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>