archiver: don't use machine variables in shared recipes

When using multiconfig with the same TMP folder we can have
races because the shared recipes like gcc-source run twice.

ARCHIVER_OUTDIR = ${ARCHIVER_TOPDIR}/${TARGET_SYS}/${PF}/
which includes TARGET_SYS and between the two different MACHINE values,
this changes  from 'arm-poky-linux-gnueabi' to 'aarch64-poky-linux'.
This leads to the task running twice, once for each multiconfig.

To solve this we need to store the shared output in a common place
for all machines and in this way the stamps will be the same for each
machine so the gcc-source will on run once regardless of the machine used.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>