riscv: Hoist linker relaxation disabling logic into Kconfig

riscv: Hoist linker relaxation disabling logic into Kconfig

Certain configurations may need to be disabled if linker relaxation is
in use, such as DWARF5 with ld.lld < 18. Hoist the logic of whether or
not linker relaxation is in use into Kconfig so decisions can be made at
configuration time.

Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20231205-riscv-restrict-dwarf5-llvm-v2-1-aedf00a382ac@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>