Remove pointless variable in win32/config_sh.PL

$int64f is always 'I64' since 6e75769d30 so there is no longer any point
in it.
Eliminate pointless win32/makefile.mk macro

As per 6e75769d30 and previous commit.
regen_config_h for VC++

(Note: Had to locally undo c69a26e800 to avoid trouble redefining stdio to
PerlIO when building in a minimal configuration with USE_PERLIO=undef!)
Define PERL_PRI[efg]ldbl and PERL_SCNfldbl in Win32 canned config files

Without PERL_PRIgldbl, the gcc build on Win32 is completely broken,
stopping at sv.c in the miniperl build since 61cd60c6c5 added a use of
PERL_PRIgldbl in its new FV_GF.
regen_config_h for GCC

(Note: Had to locally undo c69a26e800 to avoid trouble redefining stdio to
PerlIO when building in a minimal configuration with USE_PERLIO=undef!)