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>

toolchain/gcc/Config.in

@@ -2,8 +2,7 @@
 
 choice
 	prompt "GCC compiler Version" if TOOLCHAINOPTS
-	default GCC_USE_VERSION_8 if arc
-	default GCC_USE_VERSION_7
+	default GCC_USE_VERSION_8
 	help
 	  Select the version of gcc you wish to use.
 

toolchain/gcc/Config.version

@@ -2,9 +2,8 @@ config GCC_VERSION_5
 	default y if GCC_USE_VERSION_5
 	bool
 
-config GCC_VERSION_8
-	default y if GCC_USE_VERSION_8
-	default y if arc && !GCC_USE_VERSION_9
+config GCC_VERSION_7
+	default y if GCC_USE_VERSION_7
 	bool
 
 config GCC_VERSION_9
@@ -14,6 +13,6 @@ config GCC_VERSION_9
 config GCC_VERSION
 	string
 	default "5.5.0"		if GCC_VERSION_5
-	default "8.3.0"		if GCC_VERSION_8
+	default "7.4.0"		if GCC_VERSION_7
 	default "9.2.0"		if GCC_VERSION_9
-	default "7.4.0"
+	default "8.3.0"