[RISCV] Create a LinuxTargetInfo when targeting Linux

Previously, RISCV32TargetInfo or RISCV64TargetInfo were created 
unconditionally. Use LinuxTargetInfo<RISCV??TargetInfo> to ensure that the 
proper OS-specific defines are present.

This patch only adds logic to instantiate LinuxTargetInfo and leaves a TODO, 
as I'm reluctant to add logic for other targets (e.g. FreeBSD, RTEMS) until 
I've produced and tested at least one binary for that OS+target combo.

Thanks to @mgrang to reporting the issue.

llvm-svn: 324170