base: Clean up some #ifs in _format_string.

base: Clean up some #ifs in _format_string.

These were checking for gcc version 3, well below the minimum version we
support, and were hard wired to be enabled anyway. This change gets rid
of the check and the dead code on the hard wired off branch.

Also, this change cleans up the style in the surviving code and
simplifies it slightly.

Change-Id: I8df73a378f478413c111a4dea962450a37fb4092
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35977
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>