[BPF] turn on -mattr=+alu32 for cpu version v3 and later

-mattr=+alu32 has shown good performance vs. without this attribute.
Based on discussion at
  https://lore.kernel.org/bpf/1ec37838-966f-ec0b-5223-ca9b6eb0860d@fb.com/T/#t
cpu version v3 should support -mattr=+alu32.
This patch enabled alu32 if cpu version is v3, either specified by user
or probed by the llvm.

Differential Revision: https://reviews.llvm.org/D69957

lib/Target/BPF/BPFSubtarget.cpp

@@ -52,6 +52,7 @@ void BPFSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   if (CPU == "v3") {
     HasJmpExt = true;
     HasJmp32 = true;
+    HasAlu32 = true;
     return;
   }
 }

test/CodeGen/BPF/32-bit-subreg-alu.ll

@@ -1,4 +1,5 @@
 ; RUN: llc -O2 -march=bpfel -mattr=+alu32 < %s | FileCheck %s
+; RUN: llc -O2 -march=bpfel -mcpu=v3 < %s | FileCheck %s
 ;
 ; int mov(int a)
 ; {