S_require_tie_mod(): use a new stack

S_require_tie_mod(): use a new stack

RT #130861

This function is used to load a module associated with various magic vars,
like $[ and %+. Since it can be called 'unexpectedly', it should use a new
stack. The issue in this ticket was equivalent to

    my $var = '[';
    $$var;

where the symbolic dereference triggered a run-time load of arybase.pm,
which grew the stack, invalidating the SP in pp_rv2sv.

Note that most of the stuff which S_require_tie_mod() calls, such as
load_module(), will do its own PUSHSTACK(); but S_require_tie_mod() also
does a bit of stack manipulation itself.

The test case includes a magic number, 125, which happens to be the exact
size necessary to trigger a stack realloc in S_require_tie_mod(). In later
perl versions this value may well change. But it seemed too expensive
to call fresh_perl_is() 100's of times with different values of $n.

This commit also adds a SPAGAIN to pp_rv2sv on the 'belt and braces'
principle.

This commit is based on an earlier effort by Aaron Crane.