tighten Storable's recognition of tied SVs

Since commit ff44333e5a9d9dca5272bb166df463607ebd3020, being RMAGICAL
and having tie magic is not sufficient to recognise an SV as tied.
When magic is turned off for mg_set(), the RMAGICAL flag is now left on,
so that vstrings will be recognised as such.  So Storable needs to check
whether the tie magic it sees is actually in effect, by also looking at
the GMAGICAL and SMAGICAL flags.
perldiag: Correct misstatement

It's not non-alphabetic characters that are discouraged; its the
characters X where \cX doesn't map to a control that are discouraged
[perl #21442] fix fs.t failures on Win32 FAT drive

See ticket for details. This commit does not fix all issues in #21442.
[perl #21442] only check link() error codes where link() works

Skip the tests on non-NTFS.
[perl #21442] skip link() tests where the filesystem doesn't support them
[perl #21442] on Win32, skip tests that won't work on FAT when on FAT

FAT doesn't support link() and its access time resolution is only to
the day.
perldiag - Restore nitpick corrections

These were corrected in 4a70680af1 but then undone (presumably by
accident) in 727b63796f.
perldelta updates
PERL_GLOBAL_STRUCT threads issue

A test in op/threads.t roughly equivalent to this:

    use threads;
    sub f {};
    async \&f;

was randomly failing smokes under the combination of PERL_GLOBAL_STRUCT and
ASan.

This is due to the fact that, under PERL_GLOBAL_STRUCT, "global" vars
are actually allocated, and are freed as the very last thing before
exiting (later than perl_destruct()). In the code above, the created
thread is still exeecuting at this point, and so may access those "global"
vars.  ASan can detect that such a var is being accessed after being
freed; in this case due to PL_ppaddr being indexed in call_sv().

This is easily fixed using the PL_veto_cleanup mechanism; don't do those
final frees if there are still threads running.
gcc 4.9 by default does some optimizations that break perl (#121505)

Patch by Tony Cook
-fwrapv is broken prior to gcc-4.3 (googled and patched by Zefram)