[clang] [RISCV] Enable GP relaxation for Linux (PR #201265)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

> > The GP relaxation in lld only works for medlow without PIC. Linux defaults to PIE usually doesn't it?
> 
> Get it. Does the gcc in OS distros also default to PIE?

That depend on distro, but most distro are default to PIE.

Single command to check: `gcc -v 2>&1|grep default-pie -i`

https://github.com/llvm/llvm-project/pull/201265