Hi,

I've been working on a project where we cross-compile for aarch64, on an
x86_64 host. The compiler we're under orders to use (I have no say here)
is Clang.

The norm with GCC is for a `triplet-gcc` binary as CC. For Clang,
cross-compiling is achieved with a `--target=triplet` compiler flag - i.e.

`aarch64-linux-gnu-gcc foo.c`

vs

`clang --target=aarch64-linux-gnu foo.c`

Critically, this applies even for cases where CC is being used as the
linker (i.e. CCLD=clang). If no `--target` is specified, Clang will use
the native (x86_64) linker, not the linker specified in LD.

This can be worked around by throwing `-XCClinker --target=triplet` into
every linker invocation in every makefile, but it would be nice if
instead, ltmain preserved the `--target` parameter in func_mode_link
(e.g. the way it does for `--sysroot`)