bpf: Hook target feature "alu32" with LLVM

LLVM has supported a new target feature "alu32" which could be enabled or
disabled by "-mattr=[+|-]alu32" when using llc.

This patch link Clang with it, so it could be also done by passing related
options to Clang, for example:

  -Xclang -target-feature -Xclang +alu32

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325996