Various fixes for GDB's data directory.

• Backport a fix from master to fix the setting of the default auto-load paths.
• Switch to using the default data directory (PREFIX/share/gdb) instead of a version specific directory. This allows gdb to use auto-load scripts installed by other packages to PREFIX/share/gdb/auto-load. A couple of existing packages already do this such as libglib and libgobject.
• Add symlinks for gdb.1 and kgdb.1 manpages without version numbers. This makes 'man gdb' and 'man kgdb' work on 12.0 which doesn't have these manpages in the base system.
• Precompile the installed python scripts and add then to the packing list. This avoids leaving .pyc files around after the package is deleted if gdb (or kgdb) is run as root.
• Bump PORTREVISION.

Reviewed by: pizzamig (maintainer)
Differential Revision: https://reviews.freebsd.org/D18476