handle PACKAGE_ARCH change in an amend.inc

handle PACKAGE_ARCH change in an amend.inc

* setting PACKAGE_ARCH = "${MACHINE_ARCH}" in an amend.inc actually
leads to a package with the correct machine arch but the
work/stage/stamp dirs are not correct because MULTIMACH_ARCH is not
updated after amend.inc is parsed.

* this patch :
- factorizes the code which compute MULTIMACH_ARCH in base.bbclass
- updates MULTIMACH_ARCH if required by an amend.inc

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Chris Larson <chris_larson@mentor.com>
Acked-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Denys Dmytriyenko <denis@denix.org>