Reword error message of reserved AVR registers

Those are reserved as per the GCC (and thus LLVM) ABI, which is distinct from an
issue. The rewording was requested in this [review].

[review]: rust-lang#131323 (comment)

Author: jfrimmel

compiler/rustc_target/src/asm/avr.rs

@@ -105,7 +105,7 @@ def_regs! {
         #error = ["SP", "SPL", "SPH"] =>
             "the stack pointer cannot be used as an operand for inline asm",
         #error = ["r0", "r1", "r1r0"] =>
-            "r0 and r1 are not available due to an issue in LLVM",
+            "LLVM reserves r0 (scratch register) and r1 (zero register)",
             // If this changes within LLVM, the compiler might use the registers
             // in the future. This must be reflected in the set of clobbered
             // registers, else the clobber ABI implementation is *unsound*, as