make enabling LOWP stages simpler
make enabling LOWP stages simpler

This method is a little simpler macro-wise,
and makes it easier to guard new lowp stages:

  LOWP(foo)
  LOWP(bar)
  #ifndef SK_LEGACY_LOWP_BAZ
    LOWP(baz)
  #endif

Change-Id: I06392f5cf7a04651e7bf47e79f10f7da8520f5ab
Reviewed-on: https://skia-review.googlesource.com/63141
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,google/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc