abi: add a rust-preserve-none calling convention

This is the conceptual opposite of the rust-cold calling convention and
is particularly useful in combination with the new `explicit_tail_calls`
feature.

For relatively tight loops implemented with tail calling (`become`) each
of the function with the regular calling convention is still responsible
for restoring the initial value of the preserved registers. So it is not
unusual to end up with a situation where each step in the tail call loop
is spilling and reloading registers, along the lines of:

    foo:
        push r12
        ; do things
        pop r12
        jmp next_step

This adds up quickly, especially when most of the clobberable registers
are already used to pass arguments or other uses.

I was thinking of making the name of this ABI a little less LLVM-derived
and more like a conceptual inverse of `rust-cold`, but could not come
with a great name (`rust-cold` is itself not a great name: cold in what
context? from which perspective? is it supposed to mean that the
function is rarely called?)

src/abi.rs

@@ -243,6 +243,8 @@ impl<'gcc, 'tcx> FnAbiGccExt<'gcc, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
 pub fn conv_to_fn_attribute<'gcc>(conv: CanonAbi, arch: &Arch) -> Option<FnAttribute<'gcc>> {
     let attribute = match conv {
         CanonAbi::C | CanonAbi::Rust => return None,
+        // gcc/gccjit does not have anything for this.
+        CanonAbi::RustPreserveNone => return None,
         CanonAbi::RustCold => FnAttribute::Cold,
         // Functions with this calling convention can only be called from assembly, but it is
         // possible to declare an `extern "custom"` block, so the backend still needs a calling