Support the fs_base and gs_base registers on i386.

Support the fs_base and gs_base registers on i386.

As on amd64, these registers hold the base address of the fs and gs
segments, respectively.  For i386 these two registers are 32 bits.

gdb/ChangeLog:

	* amd64-fbsd-nat.c (amd64_fbsd_nat_target::read_description):
	Update calls to i386_target_description to add 'segments'
	parameter.
	* amd64-tdep.c (amd64_init_abi): Set tdep->fsbase_regnum.  Don't
	add segment base registers.
	* arch/i386.c (i386_create_target_description): Add 'segments'
	parameter to enable segment base registers.
	* arch/i386.h (i386_create_target_description): Likewise.
	* features/i386/32bit-segments.xml: New file.
	* features/i386/32bit-segments.c: Generate.
	* i386-fbsd-nat.c (i386_fbsd_nat_target::read_description): Update
	call to i386_target_description to add 'segments' parameter.
	* i386-fbsd-tdep.c (i386fbsd_core_read_description): Likewise.
	* i386-go32-tdep.c (i386_go32_init_abi): Likewise.
	* i386-linux-tdep.c (i386_linux_read_description): Likewise.
	* i386-tdep.c (i386_validate_tdesc_p): Add segment base registers
	if feature is present.
	(i386_gdbarch_init): Pass I386_NUM_REGS to set_gdbarch_num_regs.
	Add 'segments' parameter to call to i386_target_description.
	(i386_target_description): Add 'segments' parameter to enable
	segment base registers.
	(_initialize_i386_tdep) [GDB_SELF_TEST]: Add 'segments' parameter
	to call to i386_target_description.
	* i386-tdep.h (struct gdbarch_tdep): Add 'fsbase_regnum'.
	(enum i386_regnum): Add I386_FSBASE_REGNUM and I386_GSBASE_REGNUM.
	Define I386_NUM_REGS.
	(i386_target_description): Add 'segments' parameter to enable
	segment base registers.

gdb/gdbserver/ChangeLog:

	* linux-x86-tdesc.c (i386_linux_read_description): Update call to
	i386_create_target_description for 'segments' parameter.
	* lynx-i386-low.c (lynx_i386_arch_setup): Likewise.
	* nto-x86-low.c (nto_x86_arch_setup): Likewise.
	* win32-i386-low.c (i386_arch_setup): Likewise.