Fix a problem where crc32c not built on ARM64 on Chrome OS

In crc32c BUILD.gn file, it sets -march flag to armv8-a+crc+crypto
in order to support some special instructions. However, on Chrome OS,
it also sets -march flag for ARM64 which might mask the flag set by
GN file. This CL fixes this problem by passing "crc" and "crypto"
directly to clang through "-target-feature" flag. As suggested by
the reviewers, the CL changes to flag to clang only and applies to
all platforms.

CL crrev.com/c/1489392

Bug: chromium:934016
TEST: Tested on Chrome OS with tryjobs kevin64 and arm64 with testing
Change-Id: Ifb4a50308f230a7d8198f3cd4b92301e63724e6d
Reviewed-on: https://chromium-review.googlesource.com/c/1492489
Commit-Queue: Tiancong Wang <tcwang@google.com>
Auto-Submit: Tiancong Wang <tcwang@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636948}

third_party/crc32c/BUILD.gn

@@ -113,7 +113,23 @@ source_set("crc32c_arm64") {
 
   configs += [ ":crc32c_config" ]
   if (current_cpu == "arm64") {
-    cflags = [ "-march=armv8-a+crc+crypto" ]
+    if (is_clang) {
+      cflags = [
+        "-march=armv8-a",
+
+        # Some builds set -march to a different value from the above.
+        # The specific feature flags below enable the instructions we need
+        # in these cases. See https://crbug.com/934016 for example.
+        "-Xclang",
+        "-target-feature",
+        "-Xclang",
+        "+crc",
+        "-Xclang",
+        "-target-feature",
+        "-Xclang",
+        "+crypto",
+      ]
+    }
   }
 
   deps = [