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

bp2build/build_conversion_test.go

@@ -369,6 +369,7 @@ custom {
       x86_64:  { arch_paths: ["x86_64.txt"] },
       arm:  { arch_paths: ["arm.txt"] },
       arm64:  { arch_paths: ["arm64.txt"] },
+      riscv64: { arch_paths: ["riscv64.txt"] },
     },
     target: {
       linux: { arch_paths: ["linux.txt"] },
@@ -401,6 +402,10 @@ custom {
             "arm64.txt",
             "lib64.txt",
         ],
+        "//build/bazel/platforms/arch:riscv64": [
+            "riscv64.txt",
+            "lib64.txt",
+        ],
         "//build/bazel/platforms/arch:x86": [
             "x86.txt",
             "lib32.txt",