toolchain-external: adjust musl dynamic linker symlink for mips-sf

The external toolchain code has some logic to calculate the correct name
for the dynamic linker symbolic link that needs to be created when the
musl C library is being used. There was already some handling for the
mipsel+soft-float case, but not for the mips+soft-float case. Due to
this, the symbolic link was incorrectly named, and programs were
referencing an non-existing file.

Reported-by: Florent Jacquet <florent.jacquet@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>