Support gdbclient.py on riscv64

Add the path to the existing architecture-specific runtime directory,
in preparation for when lldb-server is added to the toolchain build by
Iba7efaa62e7d3cc8e3bd1080d9b4a67bc241faf2.

Test: `gdbclient.py -r surfaceflinger` connected to a riscv64 board
Change-Id: I1654378a7ba92fcd7ffe478d2b491f426655e6f1
diff --git a/scripts/gdbclient.py b/scripts/gdbclient.py
index ef32bf0..b053a48 100755
--- a/scripts/gdbclient.py
+++ b/scripts/gdbclient.py
@@ -56,6 +56,7 @@
     arch = {
         'arm': 'arm',
         'arm64': 'aarch64',
+        'riscv64': 'riscv64',
         'x86': 'i386',
         'x86_64': 'x86_64',
     }[arch]