kbuild: replace AS=clang with LLVM_IAS=1

kbuild: replace AS=clang with LLVM_IAS=1

The 'AS' variable is unused for building the kernel. Only the remaining
usage is to turn on the integrated assembler. A boolean flag is a better
fit for this purpose.

AS=clang was added for experts. So, I replaced it with LLVM_IAS=1,
breaking the backward compatibility.

Suggested-by: Nick Desaulniers <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>