riscv64/Makefile: disable gp relaxations

[badochov]

binutils 2.41 silently introduced gp relaxations which for some reason make kernel impossible to build
JIRA: RTOS-912

Description

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work
  riscv: rename sbadaddr to stval #587
  https://github.com/phoenix-rtos/phoenix-rtos-build/pull/203/files
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 723 tests  ±0   7 008 ✅ ±0   37m 9s ⏱️ +12s
  443 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit cd99881. ± Comparison against base commit cb7fddd.

♻️ This comment has been updated with latest results.

[Darchiv]

Please add a (short) description of this change (+why it is needed), so that no one has to dive into commit section.

[badochov]

Please add a (short) description of this change (+why it is needed), so that no one has to dive into commit section.

done