This got broken again due to confusion about the interaction between
the various debug/symbol/whatever variables, so let's try to clean it
up once more. So gb_SYMBOLS or any other global flag is no more.
For checking whether a build target should get symbols, use
gb_LinkTarget__symbols_enabled, which is internally controlled by
gb_ENABLE_SYMBOLS_FOR (and flags from configure, command line or
wherever affect that).

This commit breaks the debug/nodebug split for PCH files, but fixing
that is a relatively separate and complex change, so it'll be done
in another commit.

Change-Id: I6060dd38684445bb761e664344fb530386481332
Reviewed-on: https://gerrit.libreoffice.org/70369
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>