Reland "[flang] GET_COMMAND_ARGUMENT runtime implementation"

Recommit https://reviews.llvm.org/D109813 and
https://reviews.llvm.org/D109814.

This implements the second and final entry point for GET_COMMAND_ARGUMENT,
handling the VALUE, STATUS and ERRMSG parameters.

It has a small fix in that we're now using memcpy instead of strncpy
(which was a bad idea to begin with, since we're not actually interested
in a string copy).