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

Authored by yonghong-song on Nov 7 2019, 10:55 AM.

Description

[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