SBCL: implement THREAD-ATTRIBUTES only for Linux

SBCL defines SB-THREAD:THREAD-OS-TID only for Linux, and accessing it
from MacOS or Windows will result in a symbol not found error:

    ; caught ERROR:
    ;   READ error during COMPILE-FILE:
    ;
    ;     Symbol "THREAD-OS-TID" not found in the SB-THREAD package.
    ;
    ;       Line: 1716, Column: 40, File-Position: 69208
    ;
    ;       Stream: #<SB-INT:FORM-TRACKING-STREAM for "file /Users/matteolandi/my-env/opt/slime/swank/sbcl.lisp" {1001ECFC23}>
    ;;
    ;; Error compiling /Users/matteolandi/my-env/opt/slime/swank/sbcl.lisp:
    ;;   COMPILE-FILE returned NIL.
    ;;

swank/sbcl.lisp

@@ -1720,10 +1720,10 @@ stack."
     ;; sometimes the name is not a string (e.g. NIL)
     (princ-to-string (sb-thread:thread-name thread)))
 
+  #+linux
   (defimplementation thread-attributes (thread)
     (let* ((tid (sb-thread:thread-os-tid thread))
            (cpu-time
-             #+linux
              (ignore-errors
                (with-open-file (s (format nil "/proc/self/task/~d/schedstat" tid))
                  (* (parse-integer (read-line s nil nil) :junk-allowed t)