gdb: Fix regression in varobj recreation

Commit bc20e562ec0 "Fix use after free in varobj" introduced a
regression.  This commit makes sure that the varobj object does not
keeps stale references to object being freed when we unload an objfile.
This includes the "valid_block" field which is reset to nullptr if the
pointed to block is tied to an objfile being freed.

However, at some point varobj_invalidate_iter might try to recreate
varobjs tracking either floating or globals.  Varobj tracking globals
are identified as having the "valid_block" field set nullptr, but as
bc20e562ec0 might clear this field, we have lost the ability to
distinguish between varobj referring to globals and non globals.

Fix this by introducing a "global" flag which tracks if a given varobj
was initially created as tracking a global.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29426

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