[libc][NFC] add remarks to the setjmp implementation (llvm#137066)

We use naked functions to avoid compiler-generated prologue and
epilogue.

Despite GCC documentation listing this as an unsupported case for
extended asm, the generated code is not wrong as we only pass in
constant operands to extended asm.

See llvm#137055 for related
remarks.

---------

Co-authored-by: Copilot <[email protected]>

Author: SchrodingerZhu

libc/src/setjmp/x86_64/setjmp.cpp

@@ -6,6 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// We use naked functions to avoid compiler-generated prologue and epilogue.
+// Despite GCC documentation listing this as an unsupported case for extended
+// asm, the generated code is not wrong as we only pass in constant operands
+// to extended asm.
+// See https://github.com/llvm/llvm-project/issues/137055 for related remarks.
+
 #include "hdr/offsetof_macros.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"