stabilize abi_thiscall

Closes rust-lang/rust#42202, stabilizing the use of the "thiscall" ABI.

FCP was substituted by a poll, and the poll has been accepted.

Similar to prior support added for the mips430, avr, and x86 targets
this change implements the rough equivalent of clang's
[`__attribute__((interrupt))`][clang-attr] for riscv targets, enabling
e.g.

```rust
static mut CNT: usize = 0;

pub extern "riscv-interrupt-m" fn isr_m() {
    unsafe {
        CNT += 1;
    }
}
```

to produce highly effective assembly like:

```asm
pub extern "riscv-interrupt-m" fn isr_m() {
420003a0:       1141                    addi    sp,sp,-16
    unsafe {
        CNT += 1;
420003a2:       c62a                    sw      a0,12(sp)
420003a4:       c42e                    sw      a1,8(sp)
420003a6:       3fc80537                lui     a0,0x3fc80
420003aa:       63c52583                lw      a1,1596(a0) # 3fc8063c <_ZN12esp_riscv_rt3CNT17hcec3e3a214887d53E.0>
420003ae:       0585                    addi    a1,a1,1
420003b0:       62b52e23                sw      a1,1596(a0)
    }
}
420003b4:       4532                    lw      a0,12(sp)
420003b6:       45a2                    lw      a1,8(sp)
420003b8:       0141                    addi    sp,sp,16
420003ba:       30200073                mret
```

(disassembly via `riscv64-unknown-elf-objdump -C -S --disassemble ./esp32c3-hal/target/riscv32imc-unknown-none-elf/release/examples/gpio_interrupt`)

This outcome is superior to hand-coded interrupt routines which, lacking
visibility into any non-assembly body of the interrupt handler, have to
be very conservative and save the [entire CPU state to the stack
frame][full-frame-save]. By instead asking LLVM to only save the
registers that it uses, we defer the decision to the tool with the best
context: it can more accurately account for the cost of spills if it
knows that every additional register used is already at the cost of an
implicit spill.

At the LLVM level, this is apparently [implemented by] marking every
register as "[callee-save]," matching the semantics of an interrupt
handler nicely (it has to leave the CPU state just as it found it after
its `{m|s}ret`).

This approach is not suitable for every interrupt handler, as it makes
no attempt to e.g. save the state in a user-accessible stack frame. For
a full discussion of those challenges and tradeoffs, please refer to
[the interrupt calling conventions RFC][rfc].

Inside rustc, this implementation differs from prior art because LLVM
does not expose the "all-saved" function flavor as a calling convention
directly, instead preferring to use an attribute that allows for
differentiating between "machine-mode" and "superivsor-mode" interrupts.

Finally, some effort has been made to guide those who may not yet be
aware of the differences between machine-mode and supervisor-mode
interrupts as to why no `riscv-interrupt` calling convention is exposed
through rustc, and similarly for why `riscv-interrupt-u` makes no
appearance (as it would complicate future LLVM upgrades).

[clang-attr]: https://clang.llvm.org/docs/AttributeReference.html#interrupt-risc-v
[full-frame-save]: https://github.com/esp-rs/esp-riscv-rt/blob/9281af2ecffe13e40992917316f36920c26acaf3/src/lib.rs#L440-L469
[implemented by]: https://github.com/llvm/llvm-project/blob/b7fb2a3fec7c187d58a6d338ab512d9173bca987/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp#L61-L67
[callee-save]: https://github.com/llvm/llvm-project/blob/973f1fe7a8591c7af148e573491ab68cc15b6ecf/llvm/lib/Target/RISCV/RISCVCallingConv.td#L30-L37
[rfc]: rust-lang/rfcs#3246

…m-ra