Linux 3.1.5 `arch_align_stack'

Question

I'm compiling a custom Linux 3.1.5 kernel with grsecurity but I get an error at the end of sudo make:

MODPOST vmlinux.o
WARNING: modpost: Found 4919 section mismatch(es).
To see full details build your kernel with:
'make CONFIG_DEBUG_SECTION_MISMATCH=y'
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  LD      init/built-in.o
  LD      .tmp_vmlinux1
fs/built-in.o: In function `setup_arg_pages':
/usr/src/linux-3.1.5/fs/exec.c:691: undefined reference to `arch_align_stack'
fs/built-in.o: In function `create_elf_tables':
/usr/src/linux-3.1.5/fs/binfmt_elf.c:178: undefined reference to `arch_align_stack'
/usr/src/linux-3.1.5/fs/binfmt_elf.c:178: undefined reference to `arch_align_stack'
make: *** [.tmp_vmlinux1] Error 1

I'm on Xubuntu 64-bit with an Intel T4400 processor.