riscv64: enable Zba.

Cuttlefish still boots to homescreen, all instructions in the extension
are actually seen to be in use, and the bionic tests still pass.

Test: see above
Change-Id: I6373fa6245115b9b6c845db03ee73b321c07e02a
diff --git a/cc/config/riscv64_device.go b/cc/config/riscv64_device.go
index d66697c..77ee977 100644
--- a/cc/config/riscv64_device.go
+++ b/cc/config/riscv64_device.go
@@ -29,14 +29,14 @@
 		// A temporary fix for SExtWRemoval miscompilation bug.
 		"-mllvm",
 		"-riscv-disable-sextw-removal=true",
-		"-march=rv64gc_zbb",
+		"-march=rv64gc_zba_zbb",
 	}
 
 	riscv64ArchVariantCflags = map[string][]string{}
 
 	riscv64Ldflags = []string{
 		"-Wl,--hash-style=gnu",
-		"-march=rv64gc_zbb",
+		"-march=rv64gc_zba_zbb",
 	}
 
 	riscv64Lldflags = append(riscv64Ldflags,