Bug Fix: Fix GDB stub builds due to undeclared symbol

This commit fixes a build issue that occurred when compiling with
GDBSTUB support (-DGDBSTUB=ON). The issue was introduced in commit
565421a, which commented out the GDB_REG_FS_BASE and GDB_REG_GS_BASE
registers, but did not remove the code snippets that referenced them
in gdbstub_client_write_reg(). This commit comments out those code
snippets as well.

Additionally, this commit removes the fs_base and gs_base registers
from the XML file, to ensure the correct numbering of the registers
for the GDB.

src/gdbstub.c

@@ -222,8 +222,10 @@ static char      target_xml[]   = /* QEMU gdb-xml/i386-32bit.xml with modificati
             "<reg name=\"fs\" bitsize=\"16\" type=\"int32\"/>"
             "<reg name=\"gs\" bitsize=\"16\" type=\"int32\"/>"
             ""
+#if 0
             "<reg name=\"fs_base\" bitsize=\"32\" type=\"int32\"/>"
             "<reg name=\"gs_base\" bitsize=\"32\" type=\"int32\"/>"
+#endif
             ""
             "<flags id=\"i386_cr0\" size=\"4\">"
                 "<field name=\"PG\" start=\"31\" end=\"31\"/>"
@@ -548,10 +550,12 @@ gdbstub_client_write_reg(int index, uint8_t *buf)
             flushmmucache();
             break;
 
+#if 0
         case GDB_REG_FS_BASE ... GDB_REG_GS_BASE:
             /* Do what qemu does and just load the base. */
             segment_regs[(index - 16) + (GDB_REG_FS - GDB_REG_CS)]->base = *((uint32_t *) buf);
             break;
+#endif
 
         case GDB_REG_CR0 ... GDB_REG_CR4:
             *cr_regs[index - GDB_REG_CR0] = *((uint32_t *) buf);