gdb, gdbserver: propagate use of target_desc unique pointers

gdb, gdbserver: propagate use of target_desc unique pointers

Propagate the use of target_desc unique pointers further.  Basically,
avoid releasing target_desc objects, except in some spots in gdbserver
(e.g. netbsd) where we don't currently have a persistent container for
created target descriptions (and the target desc just leaks).  Introduce
const_target_desc_up to change some `const target_desc *` to a unique
pointer without loss of constness.

Architectures that use the old regformats/regdat.sh don't need to be
changed, because their target_desc objects are statically allocated in
the generated files.

I was able to built-test these native configs:

 - Linux AArch64
 - Linux ARC
 - Linux AMD64
 - Linux ARM
 - Linux LoongArch
 - Linux M68K
 - Linux Microblaze
 - Linux MIPS (32 and 64)
 - Linux or1k
 - Linux PPC (32 and 64)
 - Linux RISC-V
 - Linux s390x
 - Linux SH4
 - Linux Sparc (32 and 64)
 - Linux Xtensa
 - FreeBSD AMD64
 - NetBSD AMD64
 - Windows i686
 - Windows AMD64

For the rest, I did my best by staring at the code long enough.  I
probably missed or messed some spots, but that shouldn't be difficult to
fix.

Change-Id: I8db8790c57942edd2bfe890987157e2dc0c67879
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>