use PERL_NO_GET_CONTEXT everywhere

NYTProf.dll dropped from 76 KB to 72 KB after this change, using -O1 and
VC 2003, with perl 5.19.10, NYTP_dTHX can be defined away if NO_XSLOCKS is
added to NYTProf in the future, this commit tested NO_XSLOCKS but doesn't
turn it on.