layer.conf: Exclude gcc-cross dependencies from checksums

layer.conf: Exclude gcc-cross dependencies from checksums

gcc-cross technically depends on the C library however we can use
one toolchain for multiple different tunes within a given architecture.

Manually remove these dependencies so that gcc-cross isn't rebuild with
every tune change.

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