gcc: fix dynamic linker path for mips soft-float

gcc: fix dynamic linker path for mips soft-float

This commit updates the gcc musl patches for gcc 4.7, 4.8 and 4.9 so
that the path to the dynamic linker encoded as "program interpreter"
in the generated binaries actually matches the symbolic link installed
by musl when building for mips soft-float.

Indeed, musl installs a symlink called ld-musl-mipsel-sf.so.1, but gcc
currently generates binaries that use /lib/ld-musl-mips.so as program
interpreter.

The fix is simply the one from
https://bitbucket.org/GregorR/musl-cross/commits/825219202365, i.e
adjust MUSL_DYNAMIC_LINKER in our musl gcc patches.

Thanks to these patches:

$ ./host/usr/bin/mipsel-linux-readelf -a ./target/bin/busybox
[...]
      [Requesting program interpreter: /lib/ld-musl-mipsel-sf.so.1]
[...]

gcc 5.x doesn't need any fix because the musl patches already use the
right value.

Fixes bug #7562.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>