icecc.bbclass: Fix STAGING_BINDIR_TOOLCHAIN usage

STAGING_BINDIR_TOOLCHAIN is actually a path list, not a single path. Fix
icecc.bbclass to try all the paths in the variable instead of treating
it as a single path.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>