multilib: Fix issues with some cross-canadian toolchain sysroots

multilib: Fix issues with some cross-canadian toolchain sysroots

MACHINE = "qemumips64"
MULTILIBS = "multilib:lib64 multilib:lib32"
DEFAULTTUNE = "mips64-n32"
DEFAULTTUNE_virtclass-multilib-lib64 = "mips64"
DEFAULTTUNE_virtclass-multilib-lib32 = "mips32r2"

bitbake core-image-minimal -c populate_sdk

Results in gcc-cross-canadian-mips failing to build due to the use
of an incorrect sysroot, fix this. All nativesdk pieces should be in
the same sysroot (unprefixed).

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