toolchain-external: fix musl-based builds on ARMhf platforms

When ARCH is arm and the hard-floating-point option is on executables
expect to find the dynamic linker at /lib/ld-musl-armhf.so.1 and not
/lib/ld-musl-arm.so.1.

This patch adjusts the logic that creates the symbolic link from the
dynamic linker path to the musl C library (since musl has everything
built into a single file).

[Thomas: tweak the commit log.]

Signed-off-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>