Previuously Fuchsia-specific memory allocation logic was trying to deal
with non-page-aligned address returned from zx_vmar_map(), but it's no
longer necessary because zx_vmap_map() is guaranteed to return
page-aligned address. Also it was trying to unmap partial pages, but
zx_vmar_unmap() now requires that len is page-aligned. This change fixes
both issues by removing two redundant zx_vmar_unmap() calls. Also
deallocateExecutable() has been updated to pass page-aligned size to
zx_vmar_unmap().

This CL resolves test failures in some viz_unittests on Fuchsia.

Bug: chromium:961455
Change-Id: Ib53e46af853802679a55dacc8546c3f67a3486c7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34409Tested-by: Sergey Ulanov <sergeyu@chromium.org>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>