Add riscv64.

Otherwise we hit an unknown --arch error in ndkstubgen as soon as we try
to build an NDK library (such as libc/libdl/libm, which are the very
first libraries we need to build).

Test: treehugger
Change-Id: Id633248d1fa80eeddfd234301355931bb1309dc3
diff --git a/cc/symbolfile/__init__.py b/cc/symbolfile/__init__.py
index 9bf07f2..94c8567 100644
--- a/cc/symbolfile/__init__.py
+++ b/cc/symbolfile/__init__.py
@@ -41,6 +41,7 @@
 ALL_ARCHITECTURES = (
     Arch('arm'),
     Arch('arm64'),
+    Arch('riscv64'),
     Arch('x86'),
     Arch('x86_64'),
 )