build: fix x86 out-of-tree build without EFI

build: fix x86 out-of-tree build without EFI

We can't have a source file with the same name that exist in both the
common code and in the arch specific code for efi/. This can lead to
confusion in make and it can pick up the wrong source file. This issue
lead to a failure to build a pv-shim for x86 out-of-tree, as this is
one example of an x86 build using the efi/stub.c.

The issue is that in out-of-tree, make might find x86/efi/stub.c via
VPATH, but as the target needs to be rebuilt due to FORCE, make
actually avoid changing the source tree and rebuilt the target with
VPATH ignored, so $@ lead to the build tree where "stub.c" doesn't
exist yet so a link is made to "common/stub.c".

Rework the new common/stub.c file to have a different name than the
already existing one, by renaming the existing one. We can hide the
compat aliases that x86 uses behind CONFIG_COMPAT so a Arm build will
not have them.

Also revert the change to the rule that creates symbolic links it's
better to just recreate the link in cases where an existing file exist
or the link goes to the wrong file.

Avoid using $(EFIOBJ-y) as an alias for $(clean-files), add
"stub.c" directly to $(clean-files).

Also update .gitignore as this was also missing from the original
patch.

Fixes: 7f96859b0d00 ("xen: reuse x86 EFI stub functions for Arm")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>