RFR: Fix non-PCH build failures: x86_64, aarch64, arm32, ppc64, s390x

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/build/sbsa-non-pch/webrev.01/

This fixes non-PCH build failures on multiple platforms. The failures have this form:

/pool/buildbot/slaves/sobornost/shenandoah-jdkX/build/src/hotspot/cpu/arm/gc/shenandoah/shenandoahBarrierSetAssembler_arm.hpp:48:29:
error: ‘LIR_Assembler’ has not been declared
   void gen_pre_barrier_stub(LIR_Assembler* ce, ShenandoahPreBarrierStub* stub) {
                             ^~~~~~~~~~~~~

Instead of guessing what to forward-declare, we just include the proper headers. x86 does not need
forward declarations because of that as well.

Testing: {x86_64, aarch64, arm32, ppc64, s390x} builds

Thanks,
-Aleksey