Bug 1471147 - Enable Rust coverage on Windows. r=jmaher

--HG--
extra : rebase_source : 36f59a33597c93749f029905cc86e13ec1e4ca98
extra : histedit_source : 8897c3485dc0f42cf09a8316cf0753c4cfed473c

browser/config/mozconfigs/win64/code-coverage

@@ -29,6 +29,7 @@ fi
 
 export CFLAGS="--coverage"
 export CXXFLAGS="--coverage"
+export RUSTFLAGS="-Ccodegen-units=1 -Zprofile -Zno-landing-pads"
 
 . "$topsrcdir/build/mozconfig.common.override"
 . "$topsrcdir/build/mozconfig.clang-cl"