tools/bpf: Turn on llvm alu32 attribute by default

LLVM alu32 was introduced in LLVM7:

  https://reviews.llvm.org/rL325987
  https://reviews.llvm.org/rL325989

Experiments showed that in general performance is better with alu32
enabled:

  https://lwn.net/Articles/775316/

This patch turns on alu32 with no-flavor test_progs which is tested
most often. The flavor test at no_alu32/test_progs can be used to
test without alu32 enabled. The Makefile check for whether LLVM
supports '-mattr=+alu32 -mcpu=v3' is removed as LLVM7 should be
available for recent distributions and also latest LLVM is preferred
to run BPF selftests.

Note that jmp32 is checked by -mcpu=probe and will be enabled if the
host kernel supports it.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191022043119.2625263-1-yhs@fb.com