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>

py/persistentcode.h

@@ -98,6 +98,7 @@ enum {
     MP_NATIVE_ARCH_XTENSA,
     MP_NATIVE_ARCH_XTENSAWIN,
     MP_NATIVE_ARCH_RV32IMC,
+    MP_NATIVE_ARCH_RV64IMC,
     MP_NATIVE_ARCH_DEBUG, // this entry should always be last
 };