Update our copy of libvixl to upstream's 1.12 release.
The major benefit from QEMU's point of view is that some instructions
previously disassembled as "unimplemented (System)" are now displayed
as something more useful. It also fixes some warnings about format
strings that newer w64-mingw32 compilers were emitting.

We didn't have any local changes to libvixl so nothing needed
to be forward-ported.

Although this is a large commit (due to upstream renaming most
of the files), only a few of the files changed in this commit
are not just straight copies of upstream libvixl files:
 disas/arm-a64.cc
 disas/libvixl/Makefile.objs
 disas/libvixl/README

Note that this commit introduces some signed-unsigned comparison
warnings on the old mingw compilers. Those compilers have broken
TLS support anyway so have only ever been much use for compile tests;
anybody still using them should add -Wno-sign-compare to their
--extra-cflags.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>