[arm64] Build NDK with PAuth and BTI

This change adds a new arch named "armv8-a-branchprot". The new target
is no different from the usual "armv8-a" variant except it has the
compiler flag "-mbranch-protection=standard" set.

With this flag compiler emits Armv8.3-A Pointer Authentication and
Armv8.5-A Branch Target Identification instructions. All of these
instructions are in the hint space; therefore devices which do not
support any of these extensions would execute them as NOPs.

In terms of directory structure there is no change in the output of
build/soong/scripts/build-ndk-prebuilts.sh script.

Test: build NDK prebuilts

Change-Id: Ie52ccba5653fbf567f0b7ca312d8a8e71602ad06
diff --git a/rust/config/arm64_device.go b/rust/config/arm64_device.go
index 5066428..186e571 100644
--- a/rust/config/arm64_device.go
+++ b/rust/config/arm64_device.go
@@ -26,9 +26,10 @@
 	Arm64LinkFlags            = []string{}
 
 	Arm64ArchVariantRustFlags = map[string][]string{
-		"armv8-a":          []string{},
-		"armv8-2a":         []string{},
-		"armv8-2a-dotprod": []string{},
+		"armv8-a":            []string{},
+		"armv8-a-branchprot": []string{},
+		"armv8-2a":           []string{},
+		"armv8-2a-dotprod":   []string{},
 	}
 )