Removed the define for FCALL

This is a leftover from the PERL_OBJECT days; These days it was only
used on one spot and did nothing useful.
perl.h: STMT_START/END don't need a special case for suncc anymore
perl.h: Comment was mistakenly passed to the preprocessor

This was a typo introduced in 27da23d5
Consistently use __sun to identify SunOS

The core mostly used __sun already, but '__sun__' and 'sun' were
also present.