[cleanup] Replace ifdef'd includes with assembler-arch.h

Bug: v8:8015
Change-Id: I8099c66108d5cc1596cb9b0a00c0ecd30765cf24
Reviewed-on: https://chromium-review.googlesource.com/1174266Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55116}

src/wasm/baseline/liftoff-assembler-defs.h

@@ -5,18 +5,9 @@
 #ifndef V8_WASM_BASELINE_LIFTOFF_ASSEMBLER_DEFS_H_
 #define V8_WASM_BASELINE_LIFTOFF_ASSEMBLER_DEFS_H_
 
+#include "src/assembler-arch.h"
 #include "src/reglist.h"
 
-#if V8_TARGET_ARCH_IA32
-#include "src/ia32/assembler-ia32.h"
-#elif V8_TARGET_ARCH_X64
-#include "src/x64/assembler-x64.h"
-#elif V8_TARGET_ARCH_MIPS
-#include "src/mips/assembler-mips.h"
-#elif V8_TARGET_ARCH_MIPS64
-#include "src/mips64/assembler-mips64.h"
-#endif
-
 namespace v8 {
 namespace internal {
 namespace wasm {