doio.c: Stop semop from modifying its argument

Perl_do_semop, which implements the Perl semop function, copies its
second argument to a new struct array, which it passes to the system’s
semop function.  It then copies the contents of the struct back into
the argument’s string buffer.

Neither the Darwin nor Linux documentation says that semop modifies
the structs passed to it, and, even if it did happen, perl has never
handle it correctly.  It would have to stringify its argument forcibly
(to avoid copying back into a temporary string buffer) and also call
get-magic.  And then it would fail with a read-only argument.

Since read-only arguments have always been permitted and the copy
ing-back has never worked correctly, and since this will cause prob-
lems if we upgrade modifications to COW buffers into crashes (the
PERL_DEBUG_READONLY_COW mode I am working in), this commit removes
that code.