[foozzie] Fix architecture auto-detect with multi-arch builds

The current_cpu value was erroneously removed from the build config json.
In multi-arch builds, each toolchain subdirectory in the build-product
output emits its own build-config json, where current_cpu determines
the architecture type of the sub-build.

Correctness-fuzzer runs could wrongly determined x86 sub-builds as x64.

Bug: chromium:777285
Change-Id: I5104630cd8ebbd263d557fb29771a31a2a1d78c2
Reviewed-on: https://chromium-review.googlesource.com/737797Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48950}

BUILD.gn

@@ -837,6 +837,7 @@ action("v8_dump_build_config") {
   is_gcov_coverage = v8_code_coverage && !is_clang
   args = [
     rebase_path("$root_out_dir/v8_build_config.json", root_build_dir),
+    "current_cpu=\"$current_cpu\"",
     "dcheck_always_on=$dcheck_always_on",
     "is_asan=$is_asan",
     "is_cfi=$is_cfi",
@@ -847,6 +848,7 @@ action("v8_dump_build_config") {
     "is_tsan=$is_tsan",
     "is_ubsan_vptr=$is_ubsan_vptr",
     "target_cpu=\"$target_cpu\"",
+    "v8_current_cpu=\"$v8_current_cpu\"",
     "v8_enable_i18n_support=$v8_enable_i18n_support",
     "v8_enable_verify_predictable=$v8_enable_verify_predictable",
     "v8_target_cpu=\"$v8_target_cpu\"",

tools/foozzie/testdata/v8_build_config.json

-{"v8_target_cpu": "x64"}
+{"v8_current_cpu": "x64"}

tools/foozzie/v8_foozzie.py

@@ -127,7 +127,7 @@ def infer_arch(d8):
   executable.
   """
   with open(os.path.join(os.path.dirname(d8), 'v8_build_config.json')) as f:
-    arch = json.load(f)['v8_target_cpu']
+    arch = json.load(f)['v8_current_cpu']
   return 'ia32' if arch == 'x86' else arch