gdb: remove uses of VLA

Remove uses of VLAs, replace with gdb::byte_vector.  There might be more
in files that I can't compile, but it's difficult to tell without
actually compiling on all platforms.

Many thanks to the Linaro pre-commit CI for helping find some problems
with an earlier iteration of this patch.

Change-Id: I3e5e34fcac51f3e6b732bb801c77944e010b162e
Reviewed-by: Keith Seitz <keiths@redhat.com>