Previously -Wno-header-hygiene was added to cflags only on x64, so the
compiler was generating a lot of warnings when compiling for arm64.
Now this flag is added on all platforms.

Also removed -Wno-null-dereference - it doesn't generate any warnings.

Change-Id: Id145404dba169da64a6febdec2732aec681a11bd
Reviewed-on: https://swiftshader-review.googlesource.com/c/22028Tested-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>