x86/build: correctly record dependencies of asm-offsets.s

x86/build: correctly record dependencies of asm-offsets.s

Going through an intermediate *.new file requires telling the compiler
what the real target is, so that the inclusion of the resulting .*.d
file will actually be useful.

Fixes: 7d2d7a43d014 ("x86/build: limit rebuilding of asm-offsets.h")
Reported-by: Julien Grall <julien@xen.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>