stubdom: fix stubdom-vtpm build

stubdom: fix stubdom-vtpm build

stubdom-vtpm needs gmp and expects it under
stubdom/cross-root-x86_64/x86_64-xen-elf/lib while gmp seems to install
it under stubdom/cross-root-x86_64/x86_64-xen-elf/lib64 at least in an
openSUSE environment.

Modify gmp configure parameters to explicitly specify --libdir.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 7791790c7ab97c85306dce749c6c8eb56d1dc0da)