rust-common: Fix sstate signatures between arm hf and non-hf

rust-common: Fix sstate signatures between arm hf and non-hf

Building with MACHINE=qemuarm and MACHINE=qemux86-64 causes rust-native
to rebuild. The reason is all the ingredients into the RUST_*_SYS
variables are being tracked but that doesn't really make sense in this
case:

$ bitbake-diffsigs tmp/stamps/x86_64-linux/rust-native/1.60.0-r0.do_rust_gen_targets.sigdata.63* tmp/stamps/x86_64-linux/rust-native/1.60.0-r0.do_rust_gen_targets.sigdata.34*
NOTE: Starting bitbake server...
basehash changed from 860b8f11b10182dc5b2737f62cdb697477f714adb63eeb4d4b932d67cac8eec2 to 9379e8b9df9696e8056fec7d1534661f34dda073f6d816e241b09a2dff76ae2d
Variable rust_base_triple value changed:
@@ -36,4 +36,4 @@

 # In some cases uname and the toolchain differ on their idea of the arch name
-TUNE_FEATURES{callconvention-hard} = Set
+TUNE_FEATURES{callconvention-hard} = Unset

Switch to using the final variable value for sstate signature purposes
which avoids this issue.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>