Compile trampoline with soft-float on MIPS, to match the rest of the kernel

[kan]

Summary

Core kernel is always compiled with -msoft-float on all of our platforms,
make sure we follow the suit with trampoline as well.

In theory, hack.c code needs this treatment as well, but by being small and
empty, it does not end up as marked by either float ABI in mips and does not
produce any warnings, so let it be.

[adrian]

worked for me on mips24k!