mpy-cross: Add RISC-V RV64IMC support in MPY files.

MPY files can now hold data to be run on RV64IMC.  This can be
accomplished by passing the `-march=rv64imc` flag to mpy-cross.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tests/feature_check/target_info.py

@@ -19,6 +19,7 @@ arch = [
     "xtensa",
     "xtensawin",
     "rv32imc",
+    "rv64imc",
 ][sys_mpy >> 10]
 build = getattr(sys.implementation, "_build", "unknown")
 thread = getattr(sys.implementation, "_thread", None)