Confirmed: downgrading to gcc-5 5.3.1-14ubuntu2 from xenial fixes the issue, as does adding explicit -fno-PIE -fno-PIC to KBUILD_CFLAGS in the kernel headers Makefile.
Confirmed: downgrading to gcc-5 5.3.1-14ubuntu2 from xenial fixes the issue, as does adding explicit -fno-PIE -fno-PIC to KBUILD_CFLAGS in the kernel headers Makefile.