gdb: change default initialization for register numbers on x86

gdb: change default initialization for register numbers on x86

As defined by the enums amd64_regnum and i386_regnum the register
numbering starts at 0.
Defaults for register numbers are currently set to 0 which seems to
be the wrong default.  Set them to -1 instead.  Configure the right
register number if we find out it's supported in i386_gdbarch_init.

Similarly we don't have to set the num_*regname* variables to 0 in
i386_gdbarch_init, as it's already assigned to 0 by default.

Approved-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>