[PPC32] Support PLT calls for -msecure-plt -fpic

Summary:
In Secure PLT ABI, -fpic is similar to -fPIC. The differences are that:

* -fpic stores the address of _GLOBAL_OFFSET_TABLE_ in r30, while -fPIC stores .got2+0x8000.
* -fpic uses an addend of 0 for R_PPC_PLTREL24, while -fPIC uses 0x8000.

Reviewers: hfinkel, jhibbits, joerg, nemanjai, spetrovic

Reviewed By: jhibbits

Subscribers: adalava, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63563

llvm-svn: 364324