Revert "[wasm] Enable Liftoff by default on ia32 and x64"

This reverts commit 20f33823.

Reason for revert: Fails on win32: https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Win32/14437

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,clemensh@chromium.org

Change-Id: I0edc228a88bc74624338852826a90f1f9b6eda0b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6600, chromium:787421
Reviewed-on: https://chromium-review.googlesource.com/1112017Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53967}

src/flag-definitions.h

@@ -534,18 +534,9 @@ 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")
-// 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_BOOL(wasm_tier_up, false,
+            "enable basic tiering 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,