The linker script was included in the "$^" inputs, causing the build to
fail:
LINK build/firmware.elf
powerpc64le-linux-gnu-ld: error: linker script file 'powerpc.lds' appears multiple times
As a fix the linker script is left as a dependency of the elf, but only the
object files are linked.
Add -Wdouble-promotion and -Wfloat-conversion for most ports to ban out
implicit floating point conversions, and add extra Travis builds using
MICROPY_FLOAT_IMPL_FLOAT to uncover warnings which weren't found
previously. For the unix port -Wsign-comparison is added as well but only
there since only clang supports this but gcc doesn't.
