i386-tdep: Verify architecture before proceeding with `set/show mpx'

i386-tdep: Verify architecture before proceeding with `set/show mpx'

Make sure the architecture is `bfd_arch_i386' before handling the `set
mpx' and `show mpx' commands, avoiding the issue with `i386_mpx_enabled'
interpreting `gdbarch->tdep' according to the `struct gdbarch_tdep'
definition in i386-tdep.h while indeed in a multi-target configuration
it may have a different layout and cause GDB to crash or at least
misbehave.

	gdb/
	* i386-tdep.c (i386_mpx_info_bounds): Make sure the architecture
	is `bfd_arch_i386' before proceeding.
	(i386_mpx_set_bounds): Likewise.