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 <[email protected]>
Tested-by: Kevin Darbyshire-Bryant <[email protected]>
Signed-off-by: Paul Spooren <[email protected]>
[refactored into separate commit]
Signed-off-by: Petr Štetiar <[email protected]>