GHS: Store full path of Integrity OS directory

Fixes build project error because the OS directory was a relative
path from the binary directory.

Fixes issue during project regeneration where the `-os_dir` project option
was missing because GHS_OS_DIR was not properly stored in the cache
file.

This error was introduced by commit aa59eaac4f (GHS: Allow setting GHS
variables from toolchain file, 2021-11-15, v3.23.0-rc1~337^2).