Use -march=armv9-a for Armv9-A arch variant

Additional context from krzysio@google.com in aosp/3544179: Currently, TARGET_ARCH_VARIANT := armv9-a actually uses armv8.2-a+dotprod, which seems to have been a workaround for toolchain issues that have since been fixed.  Right now only one proto board uses this TARGET_ARCH_VARIANT internally, so it seems reasonable to correct this.

Test: llvm-objdump -d
Test: presubmit
Change-Id: I99db407eb96d82d189378fd5abd4cc80ab802f20
diff --git a/cc/config/arm64_device.go b/cc/config/arm64_device.go
index 0dcf2cf..45b1580 100644
--- a/cc/config/arm64_device.go
+++ b/cc/config/arm64_device.go
@@ -44,7 +44,7 @@
 		// On ARMv9 and later, Pointer Authentication Codes (PAC) are mandatory,
 		// so -fstack-protector is unnecessary.
 		"armv9-a": []string{
-			"-march=armv8.2-a+dotprod",
+			"-march=armv9-a",
 			"-mbranch-protection=standard",
 			"-fno-stack-protector",
 		},