Pass -hwasan-allow-ifunc when building with hwasan.

Now that the toolchain and bionic both support ifuncs in
static executables, we can turn this on. Reduces size of
walleye_hwasan-userdebug system.img by 11MB.

Change-Id: I2f19b22518153328eee3fdca79d0c851cc513c3a
diff --git a/cc/sanitize.go b/cc/sanitize.go
index e6df44f..26418ad 100644
--- a/cc/sanitize.go
+++ b/cc/sanitize.go
@@ -35,7 +35,11 @@
 	asanLdflags = []string{"-Wl,-u,__asan_preinit"}
 	asanLibs    = []string{"libasan"}
 
-	hwasanCflags = []string{"-fno-omit-frame-pointer", "-Wno-frame-larger-than=", "-mllvm", "-hwasan-create-frame-descriptions=0"}
+	// TODO(pcc): Stop passing -hwasan-allow-ifunc here once it has been made
+	// the default.
+	hwasanCflags = []string{"-fno-omit-frame-pointer", "-Wno-frame-larger-than=",
+		"-mllvm", "-hwasan-create-frame-descriptions=0",
+		"-mllvm", "-hwasan-allow-ifunc"}
 
 	cfiCflags = []string{"-flto", "-fsanitize-cfi-cross-dso",
 		"-fsanitize-blacklist=external/compiler-rt/lib/cfi/cfi_blacklist.txt"}