base: support ImmediateCrashTest on mac arm64

This platform sometimes has a spare BRK #1 at the end of
IMMEDIATE_CRASH, for secretive compiler reasons. Allow
for that here.

Bug: 1128436,1121871
Change-Id: I86cb8d015b2ec6c5b6160a6db28682faa9dc4c77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415092
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807674}

base/immediate_crash_unittest.cc

@@ -150,6 +150,18 @@ TEST(ImmediateCrashTest, ExpectedOpcodeSequence) {
     // BRK #1
     EXPECT_EQ(0XD4200020, *++it);
 
+#elif defined(OS_MAC)
+
+    // BRK #0
+    EXPECT_EQ(0XD4200000, *++it);
+    // HLT #0
+    EXPECT_EQ(0XD4400000, *++it);
+
+    // Allow, but do not require, a BRK #1 after the HLT; some clangs emit this
+    // for __builtin_unreachable() but some do not.
+    if (*++it != 0XD4200020)
+      --it;
+
 #else
 
     // BRK #0