custom-hook.m4 OPENJ9_PLATFORM_SETUP needs to support arm

[JamesKingdon]

In custom-hook.m4 OPENJ9_PLATFORM_SETUP the if statement on OPENJ9_CPU needs extending to cover arm. The following might be a starting point:

  elif test "x$OPENJ9_CPU" = xarm; then
    OPENJ9_PLATFORM_CODE=xr32
    OPENJ9_BUILDSPEC=linux_arm

[DanHeidinga]

@JamesKingdon Can you open a PR for this change?

Checkout where osx is used in the makefiles / configure files in openjdk as the same places will need to be modified for arm. I think there were 3 places that needed to be touched for OSX.

[JamesKingdon]

Addressed by #87