Reenable OpenMP by reverting the following two commits. (#15315)

Reenable OpenMP by reverting the following two commits. (#15315)

Summary:
Revert "Put back linker flag for OpenMP to prevent build break on ppc64le (#14569)"

This reverts commit a84e873bb156080ea76ab182171b1f3b4d5395f6.

Revert "Update OpenMP cmake setting for xcode 9 compiler(AppleClang 9.0) (#14473)"

This reverts commit 8901935ad42fe9bf093d1106ea43606008a4024d.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15315

Differential Revision: D13495852

Pulled By: ezyang

fbshipit-source-id: bcd3f60088b14831c53d3c171f10cd1ab6b35dee