MIPS/LD/testsuite: Fix `attr-gnu-4-10' failures with OpenBSD targets

MIPS/LD/testsuite: Fix `attr-gnu-4-10' failures with OpenBSD targets

OpenBSD targets produce ELF64 files while the pattern dump expects ELF32
output and specific header sizes.  Disable it for `mips64*-*-openbsd*'
for these targets then, removing these failures:

mips64-openbsd  -FAIL: ld-mips-elf/attr-gnu-4-10
mips64el-openbsd  -FAIL: ld-mips-elf/attr-gnu-4-10

	ld/
	* testsuite/ld-mips-elf/attr-gnu-4-10.d: Add `notarget' tag with
	`mips64*-*-openbsd*'.