[kernel][riscv] start.S code and build system setup for the kernel

RISC-V kernel fixup patch 1/12

Modify start.S to use the new physboot ELF handoff.
Put the startup code in .text.boot0 and add to the linker script to sort
   first as a convenience.
Set the start address of the kernel to -4GB.
Configure the shadow call stack.
Add tweak to boot mmu code to compile without sanitizers.

Bug: 122873
Change-Id: I33e6c272138a0b1b1ea742dfdb32e8cb401e68ce
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/822188
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Roland McGrath <mcgrathr@google.com>
Fuchsia-Auto-Submit: Travis Geiselbrecht <travisg@google.com>
4 files changed
tree: 5271024d9c7dc7da15489c4df4f274e4d895aef0
  1. boards/
  2. build/
  3. bundles/
  4. docs/
  5. examples/
  6. products/
  7. scripts/
  8. sdk/
  9. src/
  10. third_party/
  11. tools/
  12. zircon/
  13. .clang-format
  14. .clang-tidy
  15. .editorconfig
  16. .git-blame-ignore-revs
  17. .gitattributes
  18. .gitignore
  19. .gitmodules
  20. .gn
  21. .ignore
  22. .style.yapf
  23. analysis_options.yaml
  24. AUTHORS
  25. BUILD.gn
  26. CODE_OF_CONDUCT.md
  27. CONTRIBUTING.md
  28. fuchsia.code-workspace
  29. LICENSE
  30. OWNERS
  31. PATENTS
  32. pyrightconfig.json
  33. README.md
  34. rustfmt.toml
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