MIPS: Add `.insn' to ensure a text label is defined as code not data

Avoid a build error with microMIPS compilation and recent versions of
GAS which complain if a branch targets a label which is marked as data
rather than microMIPS code:

../sysdeps/mips/mips32/crti.S: Assembler messages:
../sysdeps/mips/mips32/crti.S:72: Error: branch to a symbol in another ISA mode
make[2]: *** [.../csu/crti.o] Error 1

as commit 9d862524f6ae ("MIPS: Verify the ISA mode and alignment of
branch and jump targets") closed a hole in branch processing, making
relocation calculation respect the ISA mode of the symbol referred.
This allowed diagnosing the situation where an attempt is made to pass
control from code assembled for one ISA mode to code assembled for a
different ISA mode and either relaxing the branch to a cross-mode jump
or if that is not possible, then reporting this as an error rather than
letting such code build and then fail unpredictably at the run time.

This however requires the correct annotation of branch targets as code,
because the ISA mode is not relevant for data symbols and is therefore
not recorded for them.  The `.insn' pseudo-op is used for this purpose
and has been supported by GAS since:

Wed Feb 12 14:36:29 1997  Ian Lance Taylor  <ian@cygnus.com>

	* config/tc-mips.c (mips_pseudo_table): Add "insn".
	(s_insn): New static function.
	* doc/c-mips.texi: Document .insn.

so there has been no reason to avoid it where required.  More recently
this pseudo-op has been documented, by the microMIPS architecture
specification[1][2], as required for the correct interpretation of any
code label which is not followed by an actual instruction in an assembly
source.

Use it in our crti.S files then, to mark that the trailing label there
with no instructions following is indeed not a code bug and the branch
is legitimate.

References:

[1] "MIPS Architecture for Programmers, Volume II-B: The microMIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00582,
    Revision 5.04, January 15, 2014, Section 7.1 "Assembly-Level
    Compatibility", p. 533

[2] "MIPS Architecture for Programmers, Volume II-B: The microMIPS64
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00594,
    Revision 5.04, January 15, 2014, Section 8.1 "Assembly-Level
    Compatibility", p. 623

2016-11-18  Matthew Fortune  <Matthew.Fortune@imgtec.com>
            Maciej W. Rozycki  <macro@imgtec.com>

	* sysdeps/mips/mips32/crti.S (_init): Add `.insn' pseudo-op at
	`.Lno_weak_fn' label.
	* sysdeps/mips/mips64/n32/crti.S (_init): Likewise.
	* sysdeps/mips/mips64/n64/crti.S (_init): Likewise.

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