ld: optionally emit _etext last, before .data

ld: optionally emit _etext last, before .data

So, here's my suggestion for making _init .. __etext cover .text +
.rodata (including things like the read-only exception tables) for
elf64mmix.  A quick web search gives that __etext (and friends) isn't
well defined, so each target can interpret the "end of text segment"
to their own liking.  It seems likely this change is also a better fit
than the default for other ports, at least those with .rodata after
.text in the same segment.

The presence of a separate rodata-segment is optional (and not true
for elf64mmix).  This is reflected in the name as SEPARATE_TEXT /
SEPARATE_CODE isn't considered, to keep it simple; each target has to
make sure their settings of variables make sense.

ld:
	* scripttempl/elf.sc (ETEXT_LAST_IN_RODATA_SEGMENT): New variable.
	* emulparams/elf64mmix.sh (ETEXT_LAST_IN_RODATA_SEGMENT): Define.
	* testsuite/ld-mmix/sec-1.d: Adjust.