silence warning after "Fold join to const or stringify where possible"

VC 2003
op.c(4022) : warning C4244: '=' : conversion from 'unsigned short' to
'char', possible loss of data
move POD in warnings.pm to end of file to reduce module load I/O calls

warnings.pm is the hottest file/takes the most read() calls of any
module during a make all. By moving POD to the end, ~40KB of OS read()
IO was reduced to ~16KB of OS read() IO calls. Also the parser doesn't need
to search for Perl code in the POD further lessining load time because of
the __END__ token. Filed as [perl #122955].
remove excess whitespace from warnings.pm

Some lines end with spaces, remove that, use tabs instead of spaces in code
so the perl code is less bytes to read from disk. This patch saved 183
bytes. Part of [perl #122955].
speed up building with less disk IO pod moves+__END__+misc

In Cwd.pm, dont search for pwd on Win32.

Also trim down the list of makefile suffixes on Win32 so it doesn't try
searching for av.pas and perl.f90 and hash.cbl on disk.

Add __END__ tokens to stop the last read() call on the handle which
returns 0 bytes at EOF.