[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
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