Use custom GCC toolchain

scripts/build-gcc.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+PREFIX=$(pwd)/root
+
+cd gcc
+./contrib/download_prerequisites
+cd ..
+mkdir build-gcc
+cd build-gcc
+../gcc/configure --prefix="$PREFIX" \
+                 --disable-nls \
+                 --enable-languages=c \
+                 --target=x86_64-elf \
+                 --disable-bootstrap \
+                 --without-headers \
+                 --disable-shared \
+                 --disable-threads \
+                 --disable-libssp \
+                 --disable-libquadmath \
+                 --disable-libgomp \
+                 --disable-libatomic \
+                 --disable-libmudflap \
+                 --disable-multilib
+make -j$(nproc)
+make install