Move fb_xplat_cxx.bzl to xplat/build_defs
Move fb_xplat_cxx.bzl to xplat/build_defs

Reviewed By: mzlee, ttsugriy

Differential Revision: D7152463

fbshipit-source-id: ad7ca85f225343a043e2f606c6b3bbf74f42bbcd

hammerandchisel/react-native,facebook/react-native,pandiaraj44/react-native,pandiaraj44/react-native,exponentjs/react-native,exponent/react-native,pandiaraj44/react-native,hoastoolshop/react-native,janicduplessis/react-native,exponentjs/react-native,facebook/react-native,hoastoolshop/react-native,hoangpham95/react-native,pandiaraj44/react-native,exponent/react-native,hoangpham95/react-native,exponent/react-native,exponentjs/react-native,javache/react-native,exponentjs/react-native,janicduplessis/react-native,exponent/react-native,hammerandchisel/react-native,myntra/react-native,myntra/react-native,javache/react-native,hoastoolshop/react-native,pandiaraj44/react-native,myntra/react-native,exponent/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,exponentjs/react-native,hoangpham95/react-native,exponentjs/react-native,hammerandchisel/react-native,facebook/react-native,hoangpham95/react-native,hoastoolshop/react-native,hammerandchisel/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,hoastoolshop/react-native,hammerandchisel/react-native,hoangpham95/react-native,exponent/react-native,javache/react-native,hammerandchisel/react-native,janicduplessis/react-native,myntra/react-native,janicduplessis/react-native,myntra/react-native,javache/react-native,arthuralee/react-native,hoangpham95/react-native,facebook/react-native,hammerandchisel/react-native,pandiaraj44/react-native,javache/react-native,hoangpham95/react-native,exponentjs/react-native,janicduplessis/react-native,arthuralee/react-native,exponent/react-native,arthuralee/react-native,javache/react-native,myntra/react-native,arthuralee/react-native,javache/react-native,janicduplessis/react-native,hoastoolshop/react-native,pandiaraj44/react-native,hoastoolshop/react-native,hammerandchisel/react-native,hoangpham95/react-native,myntra/react-native,arthuralee/react-native,myntra/react-native,hoastoolshop/react-native,myntra/react-native,exponentjs/react-native,janicduplessis/react-native,facebook/react-native,exponent/react-native