llvm: fix SOURCE_DIR for secondary arch.
llvm: fix SOURCE_DIR for secondary arch.

haikuports/haikuports,Begasus/haikuports,moochris/haikuports,haikuports/haikuports,haikuports/haikuports,moochris/haikuports,moochris/haikuports,haikuports/haikuports,nielx/haikuports,haikuports/haikuports,nielx/haikuports,Begasus/haikuports,miqlas/haikuports,miqlas/haikuports,Begasus/haikuports,miqlas/haikuports,Begasus/haikuports,Begasus/haikuports,nielx/haikuports,koletzky/haikuports,koletzky/haikuports,koletzky/haikuports