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>

target/i386/gdbstub.c

@@ -350,15 +350,15 @@ int x86_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)
             env->segs[R_GS].base = ldl_p(mem_buf);
             return 4;
 
-#ifdef TARGET_X86_64
         case IDX_SEG_REGS + 8:
+#ifdef TARGET_X86_64
             if (env->hflags & HF_CS64_MASK) {
                 env->kernelgsbase = ldq_p(mem_buf);
                 return 8;
             }
             env->kernelgsbase = ldl_p(mem_buf);
-            return 4;
 #endif
+            return 4;
 
         case IDX_FP_REGS + 8:
             cpu_set_fpuc(env, ldl_p(mem_buf));