Differential Revision: https://reviews.llvm.org/D50246

[RISCV] Add support for computing sysroot for riscv32-unknown-elf

Extends r338385 to allow the driver to compute the sysroot when an explicit path is not provided. This allows the linker to find C runtime files and the correct include directory for header files.

Patch by lewis-revill (Lewis Revill)

llvm-svn: 341655