better glibc i_modulo bug handling

better glibc i_modulo bug handling

pp-i-modulo code currently detects a glibc bug at runtime, at the 1st
exec of each I_MODULO op.  This is suboptimal; the bug should be
detectable early, and PL_ppaddr[I_MODULO] updated just once, before
any optrees are built.

Then, because we avoid the need to fixup I_MODULO ops in already built
optrees, we can drop the !PERL_DEBUG_READONLY_OPS limitation on the
alternative/workaround I_MODULO implementation that avoids the bug.

perl.c:

bug detection code is copied from PP(i_modulo),
into S_fixup_platform_bugs(), and called from perl_construct().
It patches Perl_pp_i_modulo_1() into PL_ppaddr[I_MODULO] when needed.

pp.c:

PP(i_modulo_0), the original implementation, is renamed to PP(i_modulo)

PP(i_modulo_1), the bug-fix workaround, is renamed _glibc_bugfix
                it is #ifdefd as before, but dropping !PERL_DEBUG_READONLY_OPS

PP(i_modulo) - the 1st-exec switcher code, is dropped

ocode.pl:

Two i_modulo entries are added to @raw_alias.
- 1st alias:  Perl_pp_i_modulo		    => 'i_modulo'
- 2nd alt:    Perl_pp_i_modulo_glibc_bugfix => 'i_modulo'

1st is a restatement of the default alias/mapping that would be
created without the line.  2nd line is then seen as alternative to the
explicit mapping set by 1st.

Alternative functions are written to pp_proto.h after the standard
Perl_pp_* list, and include #if-cond, #endif wrappings, as was
specified by 2nd @raw_alias addition.

Changes tested by inserting '1 ||' into the 3 ifdefs and bug-detection code.

TODO:

In pp_proto.h generation, the #ifdef wrapping code which handles the
alternative functions looks like it should also be used for the
non-alternate functions.  In particular, there are a handful of
pp-function prototypes that should be wrapped with #ifdef HAS_SOCKET.
That said, there have been no problem reports, so I left it alone.

TonyC: make S_fixup_platform_bugs static, porting/libperl.t was failing.