multilib: let pkg-config find architecture-independent .pc files
If a multilib package depends on an allarch recipe that installs an
architecture-independent .pc file it will not be able to find the .pc file as
the recipe gets installed into the MACHINE sysroot but pkg-config looks in the
MLPREFIX-prefixed sysroot.
Solve this by extending PKG_CONFIG_PATH in multilib environments to include the
architecture-independent path in the MACHINE sysroot
(sysroots/MACHINE/usr/share/pkgconfig/).
Signed-off-by: Ross Burton <ross.burton@intel.com>
