- Eliminates OS::ReserveRegion and OS::ReserveAlignedRegion.
- Changes OS::Allocate to take alignment parameter, reorders parameters
  to match page_allocator.
- Since the size of memory allocation can be deduced, don't return the
  amount of memory allocated.
- Changes reservation of aligned address space. Before we would reserve
  (size + alignment) rounded up to page size. This is too much, because
  maximum misalignment is (alignment - page_size).
- On Windows and Cygwin, we release an oversize allocation and
  immediately retry at the aligned address in the allocation. If we
  lose the address due to a race, we just retry.
- Clean up all the calls to OS::Allocate in codegen and tests by adding
  helper AllocateSystemPage function (allocation.h) and
  AllocateAssemblerBuffer (cctest.h).
- Changes 'assm' to 'masm' in some targets for consistency when using
  a macro-assembler.

Bug: chromium:756050
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I306dbe042cc867670fdc935abca29db074b0da71
Reviewed-on: https://chromium-review.googlesource.com/749848
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49235}