x86: support fs_base and gs_base for 32-bit inferiors on 64-bit kernels

x86: support fs_base and gs_base for 32-bit inferiors on 64-bit kernels

On Linux 64-bit kernels provide fs_base and gs_base also for 32-bit
inferiors.  Add support for those registers.

Also check for HAVE_STRUCT_USER_REGS_STRUCT_FS_BASE consistently.

Signed-off-by: Markus Metzger  <markus.t.metzger@intel.com>
gdb/
	* amd64-linux-nat.c (amd64_linux_gregset32_reg_offset): Add FS_BASE
	and GS_BASE offsets.
	* amd64-linux-tdep.c (amd64_linux_gregset_reg_offset): Check
	HAVE_STRUCT_USER_REGS_STRUCT_FS_BASE.
	(amd64_linux_read_description): Enable segments if
	HAVE_STRUCT_USER_REGS_STRUCT_FS_BASE.
	* i386-linux-tdep.c (i386_linux_gregset_reg_offset): Add -1 for
	FS_BASE and GS_BASE offsets.
	(i386_linux_read_description): Enable segments if
	HAVE_STRUCT_USER_REGS_STRUCT_FS_BASE.
	* i386-linux-tdep.h (I386_LINUX_ORIG_EAX_REGNUM): Update.

Change-Id: I030b500a7a5b087452a78c3e0545d6e6e65146aa