mmix ld: move .init (and _init) first.

mmix ld: move .init (and _init) first.

This both makes the section layout more similar to that of the general
default for ELF and fixes (makes true) an assumption that code and
rodata is located between _init and __etext, in
libgcc/config/mmix/crti.S.  Sadly, that's not actually true for ELF
(generally and for elf64mmix), where exception-tables and .rodata is
after _etext; I'm pondering what to do about that.

The original mmix simulator behavior is that memory magically appears
on access, initialized with 0, which is not preferable when chasing
bugs by throwing code the size of the gcc test-suite to the simulator.
The code in crti.S compatibly enables simulator machinery to identify
undefined memory and instead stopping the simulator with an error
(going to interactive mode for interactive runs).  See
http://gcc.gnu.org/legacy-ml/gcc-patches/2012-10/msg01871.html for
more, including the mmix-sim.ch "patch file".

This fixes only one error in the gcc testsuite,
gcc.c-torture/execute/pr20621-1.c with LTO, where for some reason
gcc/lto chooses to move (writable) data that is only used to read 0 to
.rodata.  An access (sufficiently far inside a block) in an
unregistered place is flagged as an invalid access.

The bpo-9m test that I had to adjust, actually exposes a wart: mmo
does not have the notion of symbol types (or sections) and the
test-case now has leading zeros at "Main" eventually leading to it
being misdiagnosed as being outside .text and .data, thus here mapped
to BFD as an absolute symbol.  The test is not intended to check the
mmo symbol-type machinery, so I'm just tweaking it to be
symbol-type-neutral for "Main".

Since you have to jump through hoops to see the problem, I don't think
this commit is worth putting on the 2.35-branch.

ld:
	* scripttempt/mmo.sc: Move .init first in .text output section.
	* testsuite/ld-mmix/bpo-9m.d: Adjust accordingly.