x86: fix build warning

During compile of lwm2m_client using qemu_x86, the following build
warning was noticed:
zephyr/arch/x86/core/excstub.S:132:2: warning: "/*" within comment [-Wcomment]
  /*

In commit ff42bdd0 ("debug: remove option GDB_INFO"), the comment tag
was omitted.  Fix the comment end tag.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>