Constify target_desc declarations

Constify target_desc declarations

This patch changes various global target_desc declarations to const, thereby
correcting a prominent source of ODR violations in PowerPC-related target code.
The majority of files/changes are mechanical const-ifications accomplished by
regenerating the C files in features/.

This also required manually updating mips-linux-tdep.h,  s390-linux-tdep.h,
nios2-tdep.h, s390-tdep.h, arch/ppc-linux-tdesc.h, arch/ppc-linux-common.c,
and rs6000-tdep.c.

Patch tested against the sourceware trybot, and fully regression tested against
our (Red Hat's) internal  test infrastructure on Rawhide aarch64, s390x, x86_64,
and powerpcle.

With this patch, I can finally enable LTO in our GDB package builds. [Tested
with a rawhide scratch build containing this patch.]

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=22395
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24835