[arch][x86-64] fix the default TOOLCHAIN_PATH setup
[arch][x86-64] fix the default TOOLCHAIN_PATH setup

This was broken in a recent make file change by always setting
TOOLCHAIN_PREFIX in the wrapper makefile.

nvll/lk,M1cha/lk,hollanderic/lkstuff,GCrean/lk,grub4android/lk,hollanderic/lkstuff,skabet/lk,ErikCorryGoogle/lk,srodrig1/lk,skabet/lk,srodrig1/lk,nvll/lk,littlekernel/lk,sndnvaps/lk,travisg/lk,travisg/lk,M1cha/lk,M1cha/lk,travisg/lk,nvll/lk,ErikCorryGoogle/lk,srodrig1/lk,grub4android/lk,grub4android/lk,minglun-tsai/lk,littlekernel/lk,littlekernel/lk,M1cha/lk,GCrean/lk,sndnvaps/lk,skabet/lk,nvll/lk,minglun-tsai/lk,sndnvaps/lk,GCrean/lk,srodrig1/lk,ErikCorryGoogle/lk,littlekernel/lk,hollanderic/lkstuff,M1cha/lk,GCrean/lk,sndnvaps/lk,sndnvaps/lk,minglun-tsai/lk,minglun-tsai/lk,srodrig1/lk,grub4android/lk,hollanderic/lkstuff,nvll/lk,GCrean/lk,ErikCorryGoogle/lk,skabet/lk,travisg/lk,skabet/lk,grub4android/lk