GAS: Add ECOFF `.aent' pseudo-op support

GAS: Add ECOFF `.aent' pseudo-op support

Implement the ECOFF `.aent' pseudo-op for ECOFF-style `.mdebug' section
support with ELF objects and, for consistency, also with ECOFF objects.
This is so that the same MIPS source can be assembled without and with
`.mdebug' section generation enabled.

Taking the `gas/testsuite/gas/mips/aent.s' test case source as an
example and the `mips-linux' target we have:

$ as -o aent.o aent.s
$ as -mdebug -o aent.o aent.s
aent.s: Assembler messages:
aent.s:10: Error: unknown pseudo-op: `.aent'
$

because for the !ECOFF_DEBUGGING case (which is the default) the
pseudo-op is already handled by the MIPS backend with `s_mips_ent',
however no handler is present for the opposite case.

For the MIPS target this is a functional regression introduced with
commit ecb4347adecd ("Last take: approval for MIPS_STABS_ELF killing"),
<https://sourceware.org/ml/binutils/2002-06/msg00443.html>, where
support for the `.mdebug' section was added along with its associated
`-mdebug'/`-no-mdebug' command-line options, bringing an inconsistency
between the assembly syntax supported for each of these options as far
as the `.aent' pseudo-op is concerned.

Assembly language documentation available describes the pseudo-op
respectively as follows[1]:

"
.aent name, symno Sets an alternate entry point for the current
                  procedure.  Use this information when you want
                  to generate information for the debugger.  It must
                  appear inside an .ent/.end pair."

and[2]:

"
.aent name [,symno]
     Sets an alternate entry point for the current procedure.  Use this
     information when you want to generate information for the debugger.
     This directive must appear between a pair of .ent and .end directives.
     (The optional symno is for compiler use only.  It refers to a dense
     number in a .T file (symbol table).)"

Copy the approach from `s_mips_ent' then and add `.aent' support to the
`.ent' pseudo-op handler shared between the ELF and ECOFF object file
format backends, by setting BSF_FUNCTION for the symbol requested.

References:

[1] "MIPSpro Assembly Language Programmer's Guide", Silicon Graphics,
    Inc., Document Number 007-2418-004, Section 8.1 "Op-Codes", p. 96
    <http://techpubs.sgi.com/library/manuals/2000/007-2418-004/pdf/007-2418-004.pdf>

[2] "Digital UNIX Assembly Language Programmer's Guide", Digital
    Equipment Corporation, Order Number: AA-PS31D-TE, March 1996,
    Chapter 5 "Assembler Directives", p. 5-2
    <http://h41361.www4.hpe.com/docs/base_doc/DOCUMENTATION/V40G_PDF/APS31DTE.PDF>

	gas/
	* ecoff.c (ecoff_directive_ent, add_procedure): Handle `.aent'.
	* config/obj-ecoff.c (obj_pseudo_table): Add "aent" entry.
	* config/obj-elf.c (ecoff_debug_pseudo_table): Likewise.
	* testsuite/gas/mips/aent-2.d: New test.
	* testsuite/gas/mips/aent-mdebug.d: New test.
	* testsuite/gas/mips/aent-mdebug-2.d: New test.
	* testsuite/gas/mips/mips.exp: Run the new tests.