Use c++14 to pass the verilator check (backport #3876) (#3913)

* Use c++14 to pass the verilator check (#3876)

(cherry picked from commit 62291553

)

# Conflicts:
#	svsim/src/main/scala/verilator/Backend.scala

* Resolve backport conflicts

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Co-authored-by: qimingchu <1346550347@qq.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>