signal.c: if to switch

* signal.c (ruby_signal): turn multiple `if` statements into a
  `switch`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45889 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread_*.c: constify

* thread_{pthread,win32}.c (native_cond_timedwait): constify.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e