[maven plugin] Add support for platform specific buildArgs using $os: prefix

[rbygrave]

Examples:
$linux:--GC=G1
$linux:-R:MaxGCPauseMillis=50

Linux specific build arguments. These build args are not included on another os like mac.

Example:

<buildArgs>
    <buildArg>$linux:--gc=G1</buildArg>                   <!-- only use on linux-->
    <buildArg>$linux:-R:MaxGCPauseMillis=50</buildArg>    <!-- only use on linux-->
    <buildArg>-R:MaxHeapSize=400m</buildArg>
    <buildArg>--emit build-report</buildArg>
    <buildArg>--no-fallback</buildArg>
    <buildArg>-march=compatibility</buildArg>
    <buildArg>--allow-incomplete-classpath</buildArg>
    <buildArg>--static-nolibc</buildArg>
</buildArgs>

[rbygrave]

PR to address #800