toolchain/gcc: switch to version 7 by default

Newer GCC versions are reported to improve code size on various ARM
devices, which is why newer u-boot versions require them now.
There have also been user reports of data corruption issues fixed on
MIPS by updating from GCC 5 to GCC 7.

This update was previously held up by these two MIPS compile issues,
which have since been fixed:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83496
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84790

Signed-off-by: Felix Fietkau <nbd@nbd.name>

toolchain/gcc/Config.in

@@ -3,8 +3,7 @@
 choice
 	prompt "GCC compiler Version" if TOOLCHAINOPTS
 	default GCC_USE_VERSION_7_1_ARC if arc
-	default GCC_USE_VERSION_7 if x86_64 || i386
-	default GCC_USE_VERSION_5
+	default GCC_USE_VERSION_7
 	help
 	  Select the version of gcc you wish to use.
 

toolchain/gcc/Config.version

@@ -2,15 +2,13 @@ config GCC_VERSION_7_1_ARC
 	default y if (!TOOLCHAINOPTS && arc)
 	bool
 
-config GCC_VERSION_7
-	default y if GCC_USE_VERSION_7
-	default y if (!TOOLCHAINOPTS && x86_64)
-	default y if (!TOOLCHAINOPTS && i386)
+config GCC_VERSION_5
+	default y if GCC_USE_VERSION_5
 	bool
 
 config GCC_VERSION
 	string
+	default "5.5.0"		if GCC_VERSION_5
 	default "arc-2017.09-release"   if GCC_VERSION_7_1_ARC
 	default "6.3.0"         if GCC_USE_VERSION_6
-	default "7.3.0"         if GCC_VERSION_7
-	default "5.5.0"
+	default "7.3.0"