[RISCV] Create a LinuxTargetInfo when targeting Linux

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324170 91177308-0d34-0410-b5e6-96231b3b80d8