Support gnu_unique_object symbols on MIPS.

Support gnu_unique_object symbols on MIPS.

mips_declare_object_name is missing the support for declaring symbols
as gnu_unique_object that is present in the generic
ASM_DECLARE_OBJECT_NAME in elfos.h.  I'm not aware of any
MIPS-specific reason for that support to be absent;
mips_declare_object_name predates the addition of gnu_unique_object
support and as far as I can tell this was simply an oversight when
that support was added.  This patch adds the missing support,
following the code in elfos.h.

Tested with no regressions with cross to mips-linux-gnu.  In
particular, this fixes the failure of the recently-added glibc test
elf/tst-dlopen-nodelete-reloc, which relies on the compiler generating
such symbols, for MIPS.

	* config/mips/mips.c (mips_declare_object_name)
	[USE_GNU_UNIQUE_OBJECT]: Support use of gnu_unique_object.