multilib: fix SSTATE_ARCHS for multilib usage

When building multilibs, we need to inject the multilib sstate pkgarch
into SSTATE_ARCHS so the list forms a complete search path. Add a tweak
to do this.

PACKAGE_ARCH defaults to TUNE_PKGARCH so this is equivalent and just
guards against recipes changing the value which may have other unwanted
side effects.

(From OE-Core rev: 37126ffc7ccbd3df57ebbd8e581d158f03bb3b4c)

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

meta/classes-global/sstate.bbclass

@@ -85,6 +85,7 @@ SSTATE_HASHEQUIV_FILEMAP ?= " \
 
 BB_HASHFILENAME = "False ${SSTATE_PKGSPEC} ${SSTATE_SWSPEC}"
 
+SSTATE_ARCHS_TUNEPKG ??= "${TUNE_PKGARCH}"
 SSTATE_ARCHS = " \
     ${BUILD_ARCH} \
     ${BUILD_ARCH}_${ORIGNATIVELSBSTRING} \
@@ -92,7 +93,7 @@ SSTATE_ARCHS = " \
     ${SDK_ARCH}_${SDK_OS} \
     ${SDK_ARCH}_${SDK_ARCH}-${SDKPKGSUFFIX} \
     allarch \
-    ${PACKAGE_ARCH} \
+    ${SSTATE_ARCHS_TUNEPKG} \
     ${PACKAGE_EXTRA_ARCHS} \
     ${MACHINE_ARCH}"
 SSTATE_ARCHS[vardepsexclude] = "ORIGNATIVELSBSTRING"