Support RISC-V

Patch by PkmX.

This patch makes lld recognize RISC-V target and implements basic
relocation for RV32/RV64 (and RVC). This should be necessary for static
linking ELF applications.

The ABI documentation for RISC-V can be found at:
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md.
Note that the documentation is far from complete so we had to figure out
some details from bfd.

The patch should be pretty straightforward. Some highlights:

 - A new relocation Expr R_RISCV_PC_INDIRECT is added. This is needed as
   the low part of a PC-relative relocation is linked to the corresponding
   high part (auipc), see:
   https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#pc-relative-symbol-addresses

 - LLVM's MC support for RISC-V is very incomplete (we are working on
   this), so tests are given in objectyaml format with the original
   assembly included in the comments. Once we have complete support for
   RISC-V in MC, we can switch to llvm-as/llvm-objdump.

 - We don't support linker relaxation for now as it requires greater
   changes to lld that is beyond the scope of this patch. Once this is
   accepted we can start to work on adding relaxation to lld.

Differential Revision: https://reviews.llvm.org/D39322

llvm-svn: 339364