toolchain/gcc: switch to version 8 by default

toolchain/gcc: switch to version 8 by default

Main motivation for this commit is the introduction of
`-ffile-prefix-map=` which alows reproducible build path.

Compile tested on Linux and macOS without errors on the following
targets:

 * ath79
 * imx6
 * brcm2708
 * brcm63xx
 * ixp4xx
 * ramips
 * sunxi
 * x86

Thanks to Andre for the iremap fixup.

Ref: https://reproducible-builds.org/docs/build-path/
Tested-by: Andre Heider <a.heider@gmail.com>
Tested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Paul Spooren <mail@aparcar.org>
[refactored into separate commit]
Signed-off-by: Petr Štetiar <ynezz@true.cz>