build: don't build icu with -fno-rtti

ICU should be compiled with -frtti (and it sets that flag in its gyp
file) but it was also inheriting the -fno-rtti flag from common.gypi,
breaking the build on some systems.

Fixes: https://github.com/nodejs/node/issues/8867
PR-URL: https://github.com/nodejs/node/pull/8886


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>