firmware: provide a stand alone set of headers

firmware: provide a stand alone set of headers

The current build of the firmware relies on having 32bit compatible
headers installed in order to build some of the 32bit firmware.
Usually this can be solved by using the -ffreestanding compiler option
which drops the usage of the system headers in favor of a private set
of freestanding headers provided by the compiler itself that are not
tied to libc.

However such option is broken at least in the gcc compiler provided in
Alpine Linux, as the system include path (ie: /usr/include) takes
precedence over the gcc private include path:

#include <...> search starts here:
 /usr/include
 /usr/lib/gcc/x86_64-alpine-linux-musl/10.2.1/include

And the headers in /usr/include are exclusively 64bit.

Since -ffreestanding is currently broken on at least that distro, and
for resilience against future compilers also having the option broken
provide a set of stand alone 32bit headers required for the firmware
build.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>