configure.target: overwrites known cpu's to generic

[Matthias Klose]

libstdc++'s configure.target does overwrite any "known" cpu to
generic:

- i486 is overwritten to generic

- i386 has it's own abi, it's not i486, overwritten to generic anyway

- s390 is overwritten to generic

- all other cpu's not handled in the switch are overwritten to generic
  (i.e. mips/mipsel)

checked on the branch only.

	Matthias