This commit fixes REACTOR_USE_SUBZERO flag.  The commit fde88d96 sets
REACTOR_USE_SUBZERO to `false` to disable the SubZero backend.  However,
other Android.mk uses `ifdef` to check whether the SubZero backend is
enabled.  As a result, the SubZero backend is always selected because
both `REACTOR_USE_SUBZERO := true` and `REACTOR_USE_SUBZERO := false`
are considered as defined.

This commit replaces `ifdef REACTOR_USE_SUBZERO` with
`ifeq ($(REACTOR_USE_SUBZERO),true)` to fix the problem.

Bug: b/115344057
Test: Build libEGL_swiftshader and libGLES_swiftshader for Android
Change-Id: I83f2dc5018857c630a71a1b7eae636983283783a
Reviewed-on: https://swiftshader-review.googlesource.com/c/25528Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Cody Schuffelen <schuffelen@google.com>
Tested-by: Logan Chien <loganchien@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>