prepare: allow forcing LLVM_IAS (#314)

Signed-off-by: Theo Anderson <telans@posteo.de>

customization.cfg

@@ -100,6 +100,10 @@ _cpusched=""
 # For advanced users.
 _compiler=""
 
+# Force the use of the LLVM Integrated Assembler whether using LLVM, LTO or not.
+# Set to "1" to enable.
+_llvm_ias=""
+
 # Clang LTO mode, only available with the "llvm" compiler - options are "no", "full" or "thin".
 # ! This is currently experimental and might result in an unbootable kernel - Not recommended !
 # "no: do not enable LTO"

linux-tkg-config/prepare

@@ -186,7 +186,7 @@ _set_compiler(){
 
   if [ "$_compiler" = "llvm" ]; then
     _compiler_name="-llvm"
-    llvm_opt="LLVM=1"
+    llvm_opt="LLVM=1 LLVM_IAS=${_llvm_ias:-0}"
     if [ $_basever -ge 512 ]; then
       if [[ -z "$_lto_mode" || ! "$_lto_mode" =~ ^(no|thin|full)$ ]]; then
         plain "Would you like to enable Clang Link Time Optimizations (LTO) ? It may improve the performance of the kernel."