gdb: drop support for Microblaze

gdb: drop support for Microblaze

Microblaze support was never upstreamed in gdb. We currently use a gdb
from the Xilinx Github repository, but this gdb is based on 7.6, and
has never been updated in the last 4 years. There are no other active
branches at https://github.com/Xilinx/gdb/branches.

Xilinx has a slightly newer gdb, based on 7.7, available at
https://github.com/Xilinx/meta-xilinx/tree/master/recipes-microblaze/gdb. However,
it's apparently only available in the form of stack of big patches
(https://github.com/Xilinx/meta-xilinx/tree/master/recipes-microblaze/gdb/files).

Currently, gdb on Microblaze fails to build in various conditions:

 * Against glibc, with "error: conflicting types for 'ps_lgetfpregs'"

 * Against musl, with "error: unknown type name 'elf_gregset_t'"

While those issues can probably be fixed, the fact that there is no
active upstream significantly reduces the incentive to fix those
problems.

Therefore, let's drop support for gdb on Microblaze entirely.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>