[PowerPC] Set the default PLT mode on musl to Secure PLT
[PowerPC] Set the default PLT mode on musl to Secure PLT

The musl libc only supports Secure PLT.

Patch by A. Wilcox!

Reviewed By: jhibbits

Differential Revision: https://reviews.llvm.org/D59185

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@362051 91177308-0d34-0410-b5e6-96231b3b80d8

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