android: Allow forcing softpipe

When dealing with swrast, there's two possibilities: If you have LLVM, you get
llvmpipe, which is pretty fast. If you don't, you get softpipe, which is slow,
but does have a couple nice qualities, like being smaller and not needing
executable memory for JIT.

If you're building a driver that requires LLVM like radeonsi then you need the
LLVM stub for the build to find LLVM. But for swrast, since it can mean either
softpipe/llvmpipe, you don't strictly need LLVM. So this just makes the
Android build files flexible like the Meson build files (where you can specify
-Dllvm=disabled even if LLVM is findable).

Reviewed-by: Roman Stratiienko <r.stratiienko@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13532>

android/Android.mk

@@ -45,8 +45,10 @@ MESON_GEN_PKGCONFIGS := backtrace cutils expat hardware libdrm:$(LIBDRM_VERSION)
 LOCAL_CFLAGS += $(BOARD_MESA3D_CFLAGS)
 
 ifneq ($(filter swr swrast,$(BOARD_MESA3D_GALLIUM_DRIVERS) $(BOARD_MESA3D_VULKAN_DRIVERS)),)
+ifeq ($(BOARD_MESA3D_FORCE_SOFTPIPE),)
 MESON_GEN_LLVM_STUB := true
 endif
+endif
 
 ifneq ($(filter zink,$(BOARD_MESA3D_GALLIUM_DRIVERS)),)
 LOCAL_SHARED_LIBRARIES += libvulkan