[RISCV] enable LTO support, pass some options to linker.

Summary:
1. enable LTO need to pass target feature and abi to LTO code generation
   RISCV backend need the target feature to decide which extension used in
   code generation.
2. move getTargetFeatures to CommonArgs.h and add ForLTOPlugin flag
3. add general tools::getTargetABI in CommonArgs.h because different target uses different
   way to get the target ABI.

Patch by Kuan Hsu Chen (khchen)

Reviewers: lenary, lewis-revill, asb, MaskRay

Reviewed By: lenary

Subscribers: hiraditya, dschuff, aheejin, fedor.sergeev, mehdi_amini, inglorion, asb, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, steven_wu, rogfer01, MartinMosbeck, brucehoult, the_o, dexonsmith, rkruppe, PkmX, jocewei, psnobl, benna, Jim, lenary, s.egerton, pzheng, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67409

llvm-svn: 374774