Introduce armv9-a arch variant.

Armv9 with mandatory PAC and BTI extensions.
Stack protector is disabled as it is irrelevant with PAC.

Bug: 263283855
Test: NFC
Change-Id: I2f298f21dade12824597e0a6920772a2bfc63afb
diff --git a/rust/config/arm64_device.go b/rust/config/arm64_device.go
index 186e571..ae783e8 100644
--- a/rust/config/arm64_device.go
+++ b/rust/config/arm64_device.go
@@ -30,6 +30,7 @@
 		"armv8-a-branchprot": []string{},
 		"armv8-2a":           []string{},
 		"armv8-2a-dotprod":   []string{},
+		"armv9-a":            []string{},
 	}
 )