device/linux-*: use GCC-6 for all 3.x kernels

This will take too long to compile in CI, so let's [skip ci]. Compiling this on my own instead, before merging.

Related: #103 (closed) and https://postmarketos.org/vendorkernel

device/linux-amazon-thor/APKBUILD

@@ -10,9 +10,16 @@ _flavor="amazon-thor"
 url="https://kernel.org"
 license="GPL2"
 options="!strip !check !tracedeps"
-makedepends="perl sed installkernel bash gmp-dev bc linux-headers elfutils-dev xz dtbtool"
-HOSTCC="${CC:-gcc}"
-HOSTCC="${HOSTCC#${CROSS_COMPILE}}"
+makedepends="perl sed installkernel bash gmp-dev bc linux-headers elfutils-dev xz dtbtool gcc6"
+
+# Compiler: this kernel was only tested with GCC6. Feel free to make a merge
+# request if you find out that it is booting working with newer GCCs as
+# well. See <https://postmarketos.org/vendorkernel> for instructions.
+if [ "${CC:0:5}" != "gcc6-" ]; then
+	CC="gcc6-$CC"
+	HOSTCC="gcc6-gcc"
+	CROSS_COMPILE="gcc6-$CROSS_COMPILE"
+fi
 
 # Source
 _repository="kernel_phoenix_hdx-common"