c backend: fix some inline assembly constraints

[samy-00007]

This PR adds supports for:

• + as an output constraint
• more than one symbol when a register is specified (for instance, =&{rax} did not produce valid c code)
• register as both input and clobber (fixes C backend produce incorrect code for inline assembly with input as clobber #23749)

How the last one works: when an input specifies a register which is in the clobber list, I remove said register from the clobber list and input for the input list, and add a new output, marked as +r, and creates a new variable that copy the value.

So, this zig code

pub fn main() void {
    var in: usize = 20;
    _ = ∈
    asm volatile ("":: [in] "{rax}" (in) : "rax");
    foo(in);
}

fn foo(a: usize) void {
    _ = a;
}

produces for the main function

static void main_main__1005(void) {
   uintptr_t t1;
   uintptr_t t0;
   /* file:10:2 */
   t0 = (uintptr_t)20ul;
   /* dbg_var_ptr:in */
   /* file:15:2 */
   t1 = t0;
   register uintptr_t t2 __asm("rax") = t1;
   __asm volatile("": [in]"+r"(t2)::);
   /* file:16:5 */
   t1 = t0;
   /* file:16:5 */
   main_foo__1006(t1);
   return;
}

So, if the assembly uses in as a clobber, it will only change the local variable and not the actual variable.

I have not found a better way to go through the list of clobbers than adding multiple loops ...
By the way, is there a way to add tests for that ?

Also if someone could fix the x86 backend to support %= . I believe it could as simple as adding a counter that increments each time an inline assembly block is lowered to assembly, but I am not confident enough to edit that part of the codebase.

[alexrp]

cc @jacobly0