gccrs: libproc_macro: Copy ffistring data from source

Data from source pointer were not copied correctly in the ffistring
constructor.

libgrust/ChangeLog:

	* libproc_macro/ffistring.cc (FFIString::make_ffistring):
	Add copy from source pointer.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

libgrust/libproc_macro/ffistring.cc

@@ -41,7 +41,8 @@ FFIString::make_ffistring (const std::string &str)
 FFIString
 FFIString::make_ffistring (const unsigned char *data, std::uint64_t len)
 {
-  const unsigned char *inner = new unsigned char[len];
+  unsigned char *inner = new unsigned char[len];
+  std::memcpy (inner, data, len);
   return {inner, len};
 }