RISC-V: prepend --no-relax-gp to linker arguments

[TechnoPorg]

As suggested in #t-compiler/linker > RISC-V global pointer relaxations, this prepends --no-relax-gp to the link args on RISC-V to be sure we're not performing global pointer relaxations on a target that may not support them. This is with the goal of eventually stabilizing the relax target feature, as afaict behaviour around gp is the remaining blocker.

I also have a branch where relax is enabled by default on these targets (#t-compiler/risc-v > Should `-C target-feature=+relax` become the default?), but the benefits are less clear-cut than I thought they would be, so this is a safer starting point.

In the future, this could be switched to --relax-gp on targets known to support global pointer relaxations. If people want to opt in (say, for a bare metal target where they've independently set up gp), they can just add -Clink-arg={-Wl,}--relax-gp.

r? wg-linker

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

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[TechnoPorg]

@rustbot label O-riscv