Reland "move conditions for JIT into SkVM.h"

This is a reland of 166cd52cee64fb59a1c8daad379feca2e18d723a
with iOS simulator builds disabled.

Original change's description:
> move conditions for JIT into SkVM.h
>
> This makes it easier to keep in sync with the implementation in
> SkVM.cpp, and it's easier to handle here once than in each build system.
>
> Change-Id: I429f90cfc16f35a0d7e0b0fde176bc013aa7db18
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310263
> Commit-Queue: Mike Klein <mtklein@google.com>
> Commit-Queue: Herb Derby <herb@google.com>
> Auto-Submit: Mike Klein <mtklein@google.com>
> Reviewed-by: Herb Derby <herb@google.com>

Change-Id: I82837c3d959769f26197f2402d3b911e71a35675
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310465
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 2b2e47a..64204f1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1016,8 +1016,8 @@
 }
 
 optional("skvm_jit") {
-  enabled = skia_enable_skvm_jit
-  public_defines = [ "SKVM_JIT" ]
+  enabled = skia_enable_skvm_jit_when_possible
+  public_defines = [ "SKVM_JIT_WHEN_POSSIBLE" ]
   if (skia_vtune_path != "") {
     public_defines += [ "SKVM_JIT_VTUNE" ]
     public_include_dirs = [ "$skia_vtune_path/include" ]