target/i386: Fix handling of k_gs_base register in 32-bit mode in gdbstub

gdb-xml/i386-32bit.xml includes the k_gs_base register too, so we have to
handle it even if TARGET_X86_64 is not defined.  This is already done in
x86_cpu_gdb_read_register, but not in x86_cpu_gdb_write_register where the
incorrect return value causes all registers after it to be clobbered.

Fixes https://bugs.launchpad.net/qemu/+bug/1857640

.

Signed-off-by: Marek Dolata <mkdolata@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>