Kbuild: LLVMLinux: Disable the use of the Integrated Assembler when compiling with Clang

Globally disable the use of the Clang Integrated Assembler (IA). Use gas
instead. IA doesn't support the gas assembly extensions used in the kernel
code.

Author: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>