Complement commit a43942db ("LD/ELF: Unify STB_GNU_UNIQUE handling")
and use `supports_gnu_unique' with the `unique_symbol' and `type' tests,
fixing failures like:

.../binutils/testsuite/binutils-all/unique.s: Assembler messages:
.../binutils/testsuite/binutils-all/unique.s:2: Error: symbol type "gnu_unique_object" is supported only by GNU targets

ERROR: .../binutils/testsuite/binutils-all/unique.s: assembly failed
UNRESOLVED: ar unique symbol in archive
.../binutils/ar -s -r -c tmpdir/artest.a tmpdir/unique.o
Executing on host: .../binutils/ar -s -r -c tmpdir/artest.a tmpdir/unique.o   (timeout = 300)
.../binutils/ar: tmpdir/unique.o: No such file or directory

FAIL: ar unique symbol in archive

and:

.../gas/testsuite/gas/elf/type.s: Assembler messages:
.../gas/testsuite/gas/elf/type.s:30: Error: symbol type "gnu_unique_object" is supported only by GNU targets
../as-new: BFD (GNU Binutils) 2.26.51.20160628 internal error, aborting at .../gas/write.c:608 in size_seg

../as-new: Please report this bug.

.../gas/testsuite/../../binutils/readelf -s dump.o | grep "1 *\[FIONTCU\]" > dump.out
Executing on host: sh -c {.../gas/testsuite/../../binutils/readelf -s dump.o >readelf.out 2>gas.stderr}  /dev/null  (timeout = 300)
readelf: Error: dump.o: Failed to read file's magic number
FAIL: elf type list

on MIPS/FreeBSD targets:

mips-freebsd  -FAIL: ar unique symbol in archive
mips-freebsd  -FAIL: elf type list
mips64-freebsd  -FAIL: ar unique symbol in archive
mips64-freebsd  -FAIL: elf type list
mips64el-freebsd  -FAIL: ar unique symbol in archive
mips64el-freebsd  -FAIL: elf type list
mipsel-freebsd  -FAIL: ar unique symbol in archive
mipsel-freebsd  -FAIL: elf type list

	binutils/
	* testsuite/binutils-all/ar.exp: Use `supports_gnu_unique' with
	the `unique_symbol' test.

	gas/
	* testsuite/gas/elf/elf.exp: Use `supports_gnu_unique' with the
	`type' test.