malloc: Refactor TAG_ macros to avoid indirection

This does not change behaviour, just removes one layer of indirection
in the internal memory tagging logic.

Use tag_ and mtag_ prefixes instead of __tag_ and __mtag_ since these
are all symbols with internal linkage, private to malloc.c, so there
is no user namespace pollution issue.

Reviewed-by: DJ Delorie <dj@redhat.com>