Fix generation of position-independent code.

The new module discovery scheme requires the following section order:

.minfo_beg
.minfo
.minfo_end

This works for non-PIC code because the segments have the same attributes.
However, if -relocation-model=pic is passed to ldc2 then the .minfo section
becomes writeable and the sequence of sections is changed.

The quick fix is to mark the data always as writeable. Then all sections are
always writeable.

I think a better solution would be to base this in the used relocation model.
But this information is currently only available in the driver.

This fixes the failure in test case runnable/eh2.d