This will allow targets to create a target-specific MCExpr for referring
to code, bypassing any use of trampolines in the presence of a c18n
runtime.

For non-c18n runtimes this is the same as R_RISCV_CHERI_CAPABILITY, but
for c18n runtimes this marks capabilities that must not be wrapped in a
trampoline as they must point directly to the code in question.

…ELATIVE

Note that we only use R_RISCV_FUNC_RELATIVE for CheriABI binaries as
it's redundant for plain RISC-V and we would not be able to produce
binaries that work with existing upstream RISC-V runtimes.

Otherwise linking with --apply-dynamic-relocs (as is done by U-Boot, or
due to -z rel which we don't support) when --check-dynamic-relocs is
enabled (which is the default in assertions builds) will give an
internal linker error when such a relocation is emitted.

Fixes: 2d52aa8 ("[ELF][RISCV] Support R_RISCV_CHERI_CAPABILITY_CODE and R_RISCV_FUNC_RELATIVE")