ensure PL_sawampersand is exported.

Commit 1a904fc88069e249a4bd0ef196a3f1a7f549e0fe disabled
PL_sawampersand by default, since it was redundant with COW enabled.
However the mechanism to optionally re-enable it (PERL_SAWAMPERSAND)
didn't actually export the var, due to a typo.

With the new default of disabling COW but enabling PL_sawampersand,
this broke perl on builds where exports matters (and broke linux in
non-threaded builds, where porting/globvar.t detects this).