rust-cross: Replace TARGET_ARCH with TUNE_PKGARCH

rust-cross-* imported from meta-rust has incorrect signatures,
depending on MACHINEOVERRIDES making it effectively MACHINE_ARCH
as shown by sstate-diff-machines.sh:

openembedded-core/scripts/sstate-diff-machines.sh --tmpdir=tmp-glibc \
--machines="qemuarm64 qemuarm64copy" --targets=rust-cross-aarch64-glibc \
--analyze

 === Comparing signatures for task do_configure.sigdata between
 qemuarm64 and qemuarm64copy ===
ERROR: gcc-runtime different signature for task do_configure.sigdata
between qemuarm64 and qemuarm64copy
NOTE: Starting bitbake server...
Hash for dependent task gcc/gcc-runtime_11.2.bb:do_prepare_recipe_sysroot
changed from
da4ebf1b272cb73153145a0a95e6438d2955ae2d36f84db10f6880b2781ec331 to
47a0ebb7a88c9f896fb9dbce269f575ab8a6faabb2b9e62d164be6e71c5e4e40
Unable to find matching sigdata for
openembedded-core/meta/recipes-devtools/gcc/gcc-runtime_11.2.bb:
do_prepare_recipe_sysroot with hashes
da4ebf1b272cb73153145a0a95e6438d2955ae2d36f84db10f6880b2781ec331 or
47a0ebb7a88c9f896fb9dbce269f575ab8a6faabb2b9e62d164be6e71c5e4e40

The following patch takes TUNE_PKGARCH into consideration instead
of TARGET_ARCH and results in signatures as expected.

[YOCTO #14613]

RP: Added maintainer.inc corresponding change
Signed-off-by: Pgowda <pgowda.cve@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>