tools/nolibc: make the "headers" target install all supported archs

tools/nolibc: make the "headers" target install all supported archs

The efforts we go through by installing a single arch are counter
productive when the base directory already supports them all, and
the arch-specific files are really small. Let's make the "headers"
target simply install headers for all supported archs and stop
trying to build a hybrid "arch.h" file on the fly, to instead keep
the generic one. Now the same nolibc headers installation will be
usable with any arch-specific uapi installation.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>