universal.c: Ignore SvIsCOW in XS_Internals_SvREADONLY

universal.c: Ignore SvIsCOW in XS_Internals_SvREADONLY

SvIsCOW used to check SVf_READONLY|SVf_FAKE.  e3918bb703ca changed
that, but did not change the assumptions that code already made (that
there could be not truly read-only COWs.

Now SvREADONLY actually means read-only, so Internals::SvREADONLY
should not be saying that read-ony COWs are not, nor does it need to
flatten COWs when making them read-only.  Hence, locking hash values
no longer has a speed and memory hit if that hash contains COWs.

Part of the code is left in place for PERL_OLD_COPY_ON_WRITE, to avoid
making read-only COWs under that configuration.  See the previous com-
mit for why.