asm: Add a kreg0 register class on x86 which includes k0

Previously we only exposed a kreg register class which excludes the k0
register since it can't be used in many instructions. However k0 is a
valid register and we need to have a way of marking it as clobbered for
clobber_abi.

Fixes #94977

compiler/rustc_span/src/symbol.rs

@@ -803,6 +803,7 @@ symbols! {
         keyword,
         kind,
         kreg,
+        kreg0,
         label,
         label_break_value,
         lang,