kvmtool: Makefile: remove LDFLAGS from guest_init linking

Looking back at the HEAD from a few commits ago, it's obvious that
using the LDFLAGS variable for linking the guest_init binary was
rather pointless, as it was zeroed in the beginning and then never
set.

As guest_init is a rather special binary that does not cope well with
arbitrary linker flags, let's reinstantiate the previous state by
removing the LDFLAGS variable from those linking steps. This allows
LDFLAGS to be used for linking the actual kvmtool binary only and
helps to re-merge commit d0e2772b ("Makefile: allow overriding
CFLAGS on the command line").

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>