m2, build: Use LDLFAGS for mklink

m2, build: Use LDLFAGS for mklink

When trying to bootstrap current trunk on macOS 14.0 beta 3 with Xcode
15 beta 4, the build failed running mklink in stage 2:

unset CC ; m2/boot-bin/mklink -s --langc++ --exit --name m2/mc-boot/main.cc
/vol/gcc/src/hg/master/darwin/gcc/m2/init/mcinit
dyld[55825]: Library not loaded: /vol/gcc/lib/libstdc++.6.dylib

While it's unclear to me why this only happens on macOS 14, the problem
is clear: unlike other C++ executables, mklink isn't linked with
-static-libstdc++ which is passed in from toplevel in LDFLAGS.

This patch fixes that and allows the build to continue.

Bootstrapped on x86_64-apple-darwin23.0.0, i386-pc-solaris2.11, and
sparc-sun-solaris2.11.

2023-07-11  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/m2:
	* Make-lang.in (m2/boot-bin/mklink$(exeext)): Add $(LDFLAGS).