cranelift-object: Support marking data object as used

[bjorn3]

Feature

See title.

Benefit

This is required for implementing #[used(linker)] in cg_clif. This attribute is equivalent to attribute((used)) in C. And on all non-ELF platforms it is the default. when #[used] is used. On ELF platforms it may become the default in the future.

Implementation

Add a flag to DataDescription and then set the right symbol flag for the given object file format.

[Noratrieb]

Note that #[used(linker)] is __attribute__((used,retain)), not __attribute__((used)), which is just #[used(compiler)].

	Compiler	Linker
GCC C	__attribute__((used))	__attribute__((retain)) (note that you need both)
Rust	#[used(compiler)]	#[used(linker)]
LLVM	@llvm.compiler.used	@llvm.used
ELF	-	SHT_GNU_RETAIN ("R" in Assembly)