configure: triplet: accept i486 CPU (and more), more vendors and OSes

configure: triplet: accept i486 CPU (and more), more vendors and OSes

Gentoo has interesting ports:

  - freebsd with following triplet:
        ./configure --build=i686-gentoo-freebsd8 --host=i686-gentoo-freebsd8 --host=i686-gentoo-freebsd8
    should be recognized as 'i386-unknown-freebsd'
  - 'pc' vendor along with non-'i386' is very common:
        ./configure --build=i486-pc-linux-gnu --i486-pc-linux-gnu --host=i486-pc-linux-gnu
        ./configure --build=i686-pc-linux-gnu --i686-pc-linux-gnu --host=i686-pc-linux-gnu
    should be recognized as 'i386-unknown-linux'

Patch adds:

  - 'pc' vendor                 (maps to 'unknown')
  - 'gentoo' vendor             (maps to 'unknown')
  - 'i486', 'i586', 'i686' CPUs (maps to 'i386')
  - 'freebsd8' OS               (maps to 'freebsd')