Add support for linker-type-specific flags

r332090 added a LINKER_TYPE test to add the --no-rosegment flag when
linking the i386 loader components with lld. Instead, introduce a
general mechanism for setting LDFLAGS for a specific linker type,
and use it for --no-rosegment.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14998