Use Cargo's target directory sharing.

This speeds up `./mach build --dev` followed by `./mach build-cef` by a
large amount, and also speeds up other build combos found in our CI.

ports/cef/Cargo.toml

@@ -12,7 +12,6 @@ crate-type = ["dylib"]
 log = "*"
 url = "*"
 libc = "*"
-objc = "0.1"
 
 [dependencies.servo]
 path = "../../components/servo"
@@ -72,6 +71,7 @@ git = "https://github.com/servo/rust-stb-image"
 git = "https://github.com/servo/gleam"
 
 [target.x86_64-apple-darwin.dependencies]
+objc = "0.1"
 cocoa = "*"
 core-foundation = "*"
 core-graphics = "*"

ports/gonk/README.md

@@ -51,7 +51,7 @@ Run `./mach build-gonk` from the root directory
 
 ## Copy the files to the Flame
 
-To reduce the size of libmozjs.so (`ports/gonk/target/arm-linux-androideabi/build/mozjs-sys-*/out/libmozjs.so`),
+To reduce the size of libmozjs.so (`target/arm-linux-androideabi/build/mozjs-sys-*/out/libmozjs.so`),
 you can run `strip` on it. Use the one in your toolchain (`$ANDROID_TOOLCHAIN/bin/arm-linux-androideabi-strip libmozjs.so`).
 
 Make sure the device is on, connected to wifi, with high or no screen timeout.
@@ -64,7 +64,7 @@ adb remount
 adb push /path/to/stripped/mozjs.so system/lib
 
 # Copy b2s
-adb push ports/gonk/target/arm-linux-androideabi system/bin
+adb push target/arm-linux-androideabi system/bin
 
 # Copy resources
 adb shell mkdir sdcard/servo