Don't use %rcx directly with CoffTlsGetAddr (#5278)

Avoid naming %rcx as written by the CoffTlsGetAddr pseudo-instruction in the x64 backend, and instead emit a fixed-def constraint for a fresh VReg and %rcx.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -3195,10 +3195,18 @@ pub(crate) fn emit(
             sink.put1(0x17);
         }
 
-        Inst::CoffTlsGetAddr { ref symbol, dst } => {
+        Inst::CoffTlsGetAddr {
+            ref symbol,
+            dst,
+            tmp,
+        } => {
             let dst = allocs.next(dst.to_reg().to_reg());
             debug_assert_eq!(dst, regs::rax());
 
+            // tmp is used below directly as %rcx
+            let tmp = allocs.next(tmp.to_reg().to_reg());
+            debug_assert_eq!(tmp, regs::rcx());
+
             // See: https://gcc.godbolt.org/z/M8or9x6ss
             // And: https://github.com/bjorn3/rustc_codegen_cranelift/issues/388#issuecomment-532930282