libpayload: Move GDB functions to stdlib.h

libpayload: Move GDB functions to stdlib.h

When GDB support is compiled in, halt() in libpayload will call
gdb_enter(). halt() is defined in <stdlib.h> and gdb_enter() in
<libpayload.h>. Usually files just include <libpayload.h> so this is not
a problem, but in some situatons a payload may just include <stdlib.h>
(or a file including it like <assert.h>), leading to an undeclared
identifier here. Move the GDB functions to <stdlib.h> to solve this.

Change-Id: I7b23b8ac9cd302aa6ef96f24565130490ac40071
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/25730
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>