Reland^2 "[wasm] Enable Liftoff by default on ia32 and x64"

This is a reland of 20f33823.
The flakes that lead to the revert are hopefully fixed with
https://crrev.com/c/1118171.

Original change's description:
> [wasm] Enable Liftoff by default on ia32 and x64
>
> R=titzer@chromium.org, hablich@chromium.org
>
> Bug: v8:6600, chromium:787421
> Change-Id: Ia8ae56ddef3b27b0721d5a66ff19abe098a2c6ca
> Reviewed-on: https://chromium-review.googlesource.com/1109899
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Reviewed-by: Michael Hablich <hablich@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53965}

TBR=titzer@chromium.org, hablich@chromium.org

Bug: v8:6600, chromium:787421
Change-Id: Icfc6f5c7701f18891ede34edb2bc22a7c01c8d6d
Reviewed-on: https://chromium-review.googlesource.com/1118485Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54099}

src/flag-definitions.h

@@ -534,9 +534,18 @@ DEFINE_UINT(wasm_max_mem_pages, v8::internal::wasm::kV8MaxWasmMemoryPages,
             "maximum memory size of a wasm instance")
 DEFINE_UINT(wasm_max_table_size, v8::internal::wasm::kV8MaxWasmTableSize,
             "maximum table size of a wasm instance")
-DEFINE_BOOL(wasm_tier_up, false,
-            "enable basic tiering up to the optimizing compiler")
+// Enable liftoff by default on ia32 and x64. More architectures will follow
+// once they are implemented and sufficiently tested.
+#if V8_TARGET_ARCH_IA32 || V8_TARGET_ARCH_X64
+DEFINE_BOOL(
+    wasm_tier_up, true,
+    "enable wasm baseline compilation and tier up to the optimizing compiler")
+#else
+DEFINE_BOOL(
+    wasm_tier_up, false,
+    "enable wasm baseline compilation and tier up to the optimizing compiler")
 DEFINE_IMPLICATION(future, wasm_tier_up)
+#endif
 DEFINE_IMPLICATION(wasm_tier_up, liftoff)
 DEFINE_DEBUG_BOOL(trace_wasm_decoder, false, "trace decoding of wasm code")
 DEFINE_DEBUG_BOOL(trace_wasm_decode_time, false,