gcc: add upstream patch to fix build of GCC6 with GCC7

gcc: add upstream patch to fix build of GCC6 with GCC7

Building host-gcc-initial with GCC7 on the host fails due to the
comparison of a pointer to an integer in ubsan_use_new_style_p, which
is forbidden by ISO C++:

ubsan.c:1474:23: error: ISO C++ forbids comparison between pointer and
integer [-fpermissive]
       || xloc.file == '\0' || xloc.file[0] == '\xff'

Backported from:
https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=239971

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>