For i386, gcc 4.9 under Debian (at least) updated its paths to i586-linux-gnu

(i486-linux-gnu previously). Adding this triple to the list of search.
Also impacts clang 3.4.2

Reported on the Debian bug tracking system here:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754963
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=755183

llvm-svn: 213388