target-specific way to make ld -N the default

[Doug Evans]

I wrote:
 > In embedded systems work one generally doesn't want the program headers
 > being part of the first (or any) segment and thus shouldn't have to deal
 > with "Not enough room for program headers" at all.

I found this in scripttempl/elf.sc

# if this is for an embedded system, don't add SIZEOF_HEADERS.
if [ -z "$EMBEDDED" ]; then
   test -z "${TEXT_BASE_ADDRESS}" && TEXT_BASE_ADDRESS="${TEXT_START_ADDR} + SIZEOF_HEADERS"
else
   test -z "${TEXT_BASE_ADDRESS}" && TEXT_BASE_ADDRESS="${TEXT_START_ADDR}"
fi

Clearly the linker is already _trying_ to DTRT for embedded targets
w.r.t. program headers.  It's just not going far enough.

Doing a blanket default of -N for all targets that set EMBEDDED to be
non-empty in their emulparams/foo.sh script is far too risky though
I _think_ it is the correct solution.  Maybe the powers that be
can add a new keyword REALLY_EMBEDDED :-).

In the meantime there is LDEMUL_BEFORE_PARSE.
I see the mmix port is already making an attempt at this:

static void
elfmmix_before_parse ()
{
  gld${EMULATION_NAME}_before_parse ();

  /* Make sure we don't create a demand-paged executable.  Unfortunately
     this isn't changeable with a command-line option.  It makes no
     difference to mmo, but the sections in elf64mmix will be aligned to a
     page in the linked file, which is non-intuitive.  If there's ever a
     full system with shared libraries and demand paging, you will want to
     exclude this file.  */
  config.magic_demand_paged = false;
}