toolchain-funcs.eclass: Update tc-is-softfloat for new ARM tuples

toolchain-funcs.eclass: Update tc-is-softfloat for new ARM tuples

ARM tuples will change from armv7a-hardfloat-linux-gnueabi to
armv7a-unknown-linux-gnueabihf or similar in the 17.0 profiles. The
function already treated the latter as hardfloat but this commit will
now treat ambiguous tuples such as arm-unknown-linux-gnueabi as
softfloat rather than hardfloat. This brings Gentoo in line with most
of the ARM Linux community. However, the function will now check
existing toolchains to avoid breaking existing systems, if possible.

This has been achieved by splitting the function in three,
tc-detect-is-softfloat for checking existing toolchains,
tc-tuple-is-softfloat for checking just the tuple, and the new
tc-is-softfloat that calls the first two. The output from the first
two could be compared to inform the user that they are not using a
recommended tuplet.