Makefile.toolchain: Define _GNU_SOURCE when using clang
Makefile.toolchain: Define _GNU_SOURCE when using clang

g++ defines _GNU_SOURCE, but the baremetal clang++ toolchain we're using
does not. When using libcxx + newlib, _GNU_SOURCE needs to be defined
for compilation to succeed.

BRANCH=none
BUG=b:254916723
TEST=./util/compare_build.sh -b all -j 120
     => MATCH

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: Ieda973980157ecc37c2aa1afcb35cbf62bfa3c51
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3995333
Reviewed-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec