Enable target features for riscv64 Rust builds

This CL enables the following features for riscv64 Rust builds in
Android: V, Zba, Zbb, and Zbs

Test: m rust
Bug: 300313717
Bug: 300312461
Change-Id: I8712b1850a60a05e99a3e89e602bb56d2ccf2563

rust/config/riscv64_device.go

@@ -24,8 +24,12 @@ var (
 	Riscv64RustFlags = []string{
 		"-C force-frame-pointers=y",
 	}
-	Riscv64ArchFeatureRustFlags = map[string][]string{"": {}}
+	Riscv64ArchFeatureRustFlags = map[string][]string{
-	Riscv64LinkFlags            = []string{}
+		"riscv64": {
+			"-C target-feature=+V,+Zba,+Zbb,+Zbs",
+		},
+	}
+	Riscv64LinkFlags = []string{}
 
 	Riscv64ArchVariantRustFlags = map[string][]string{"": {}}
 )