MFC r303442, r305343: remove CONSTRUCTORS from linker scripts

MFC r303442, r305343: remove CONSTRUCTORS from linker scripts

r303442: remove CONSTRUCTORS from kernel linker scripts

r305343: remove CONSTRUCTORS from MIPS uboot linker script

The linker script CONSTRUCTORS keyword is only meaningful "when linking
object file formats which do not support arbitrary sections, such as
ECOFF and XCOFF"[1] and is ignored for other object file formats.

LLVM's lld does not yet accept (and ignore) CONSTRUCTORS, so just remove
CONSTRUCTORS from the linker script as it has no effect.

[1] https://sourceware.org/binutils/docs/ld/Output-Section-Keywords.html

Reported by:	andrew
Sponsored by:	The FreeBSD Foundation