Add --trace-compile= flag to --help-hidden output. (#31072)

Since the `--trace-compile` flag is an unsupported, experimental flag, we don't want to document it in `--help`, which has forward maintainability requirements. Instead, we will put the help string in `--help-hidden`, which contains documentation for unsupported flags.

src/jloptions.c

@@ -149,7 +149,9 @@ static const char opts_hidden[] =
  " --output-unopt-bc name Generate unoptimized LLVM bitcode (.bc)\n"
  " --output-jit-bc name Dump all IR generated by the frontend (not including system image)\n"
  " --output-bc name Generate LLVM bitcode (.bc)\n"
- " --output-incremental=no Generate an incremental output file (rather than complete)\n\n"
+ " --output-incremental=no Generate an incremental output file (rather than complete)\n"
+ " --trace-compile={stdout,stderr}\n"
+ " Print precompile statements for methods compiled during execution.\n\n"
 ;
 
 JL_DLLEXPORT void jl_parse_opts(int *argcp, char ***argvp)