Convert all references to /opt/mongodbtoolchain/gdb to a versioned equivalent

The mongodbtoolchain offers an unversioned path to GDB under /opt/mongodbtoolchain/gdb which refers specifically to the v3 GDB.

Unfortunately, this breaks the encapsulation of the toolchain versioning mechanism. When we want to upgrade master from using v3 to v4, we can't repoint /opt/mongodbtoolchain/gdb to v4 because this would also upgrade GDB for older branches, which we don't really want to do.

We should remove all references to /opt/mongodbtoolchain/gdb in the server codebase and replace them with /opt/mongodbtoolchain/v3/gdb, backport this change through all stable branches, and then update the toolchain builder to no longer produce /opt/mongodbtoolchain/gdb.