Merging r347179:

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r347179 | brad | 2018-11-18 16:21:06 -0800 (Sun, 18 Nov 2018) | 4 lines

[PowerPC] Set the default PLT mode on OpenBSD/powerpc to Secure PLT.

OpenBSD/powerpc only supports Secure PLT.

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llvm-svn: 347822