sim/erc32: avoid dereferencing type-punned pointer warnings

When building the erc32 simulator I get a few warnings like this:

  /tmp/build/sim/../../src/sim/erc32/exec.c:1377:21: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
   1377 |   sregs->fs[rd] = *((float32 *) & ddata[0]);
        |                    ~^~~~~~~~~~~~~~~~~~~~~~~

The type of '& ddata[0]' will be 'uint32_t *', which is what triggers
the warning.

This commit makes use of memcpy when performing the type-punning,
which resolves the above warnings.

With this change, I now see no warnings when compiling exec.c, which
means that the line in Makefile.in that disables -Werror can be
removed.

There should be no change in behaviour after this commit.