This is a follow-up to JDK-8276174.

During the review of JDK-8276174 -- https://git.openjdk.java.net/jfx/pull/666 -- Johan noted that the build script should not override the TARGET_ARCH variable, but rather should do something similar to the JDK build and define a new variable, perhaps "MACHINE_ARCH" which would represent the flag passed to the native compilers.

This should be done consistently on all platforms.

[1] https://git.openjdk.java.net/jfx/pull/666#issuecomment-974102044