get rid of function calls in RSTRING_PTR

* process.c (rlimit_resource_type, rlimit_resource_value): get rid
  of function calls in RSTRING_PTR(), as it evaluates the argument
  twice.

* re.c (match_backref_number): ditto.

* signal.c (esignal_init, rb_f_kill, trap_signm): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47006 b2dd03c8-39d4-4d8f-98ff-823fe69b080e