config.guess: replace POSIX $( ) with classic ` ` throughout

The previous replacement of backticks with POSIX command substitutions
was ill-considered and illogical: this script recognizes many archaic
machine types that probably never had POSIX shells, therefore it needs
to be able to run successfully under pre-POSIX shells.

This patch was generated using the included GNU Awk program.

* config.guess: Revert POSIX command substitutions to classic form.
* patch-6.gawk: Store the tool that produced the automated patch.