base/gcc-common: Ensure umask setting is consistent for shared workdir

gcc has cross and target components with a shared workdir. The unpack umask
settings need to match for all of these. We need to use strings in each
case to ensure the sstate code matches them correctly.

This patch tweaks various things to ensure the change adding the unpack umask
change doesn't break the compiler builds.

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