Remove "typedef enum ..."

I noticed a few spots in GDB that use "typedef enum".  However, in C++
this isn't as useful, as the tag is automatically entered as a
typedef.  This patch removes most uses of "typedef enum" -- the
exceptions being in some nat-* code I can't compile, and
glibc_thread_db.h, which I think is more or less a copy of some C code
from elsewhere.

Tested by rebuilding.