lld silently ignores .note.GNU-stack section that requests exectuable stack

[emaste]

lld contains the comment

    // The GNU linker uses .note.GNU-stack section as a marker indicating
    // that the code in the object file does not expect that the stack is
    // executable (in terms of NX bit). If all input files have the marker,
    // the GNU linker adds a PT_GNU_STACK segment to tells the loader to
    // make the stack non-executable. Most object files have this section as
    // of 2017.

This is not entirely correct -- the presence of .note.GNU-stack does not necessarily indicate that the object file is compatible with non-exec stack. That is indeed the most common case, but the note actually indicates the object's expected stack protection -- it may have an X flag to indicate an object file that requires executable stack. See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278939 for the FreeBSD issue that prompted this.

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
...
  [14] .note.GNU-stack   PROGBITS        0000000000000000 0077d8 000000 00   X  0   0  1

Executable stacks should be exceedingly rare and I have no issue with requiring an explicit -z execstack, but I think we should at least produce an error if there's a .note.GNU-stack that specifies exec stack and -z execstack is not specified. We should not silently produce broken output.

[emaste]

See also #57009