Add riscv64-linux-android support

Add barebones riscv64-linux-android support.  This should be enough
to add riscv64-specific entries to Android.bp files, but can't
actually compile anything until there are riscv64 toolchains.

Test: arch_test.go
Change-Id: I0dcc7e797d9352dd38243be908a7f19004ff3db1
diff --git a/android/arch_test.go b/android/arch_test.go
index 6814f8a..46c018a 100644
--- a/android/arch_test.go
+++ b/android/arch_test.go
@@ -596,6 +596,7 @@
 				arm64: {
 					a:  ["arm64"],
 				},
+				riscv64: { a: ["riscv64"] },
 				x86: { a:  ["x86"] },
 				x86_64: { a:  ["x86_64"] },
 			},