Add target_float

[Zoxc]

Adds target_float as suggested in rust-lang/rfcs#1364

[rust-highfive]

r? @aturon

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[hanna-kruppe]

If flt2dec and dec2flt are cfg'd out, then bignum and diy_float can also be cfg'd out..

[Zoxc]

@rkruppe Tests require libtest which require librand and libstd which require floats, so that won't be very beneficial until libstd can build without floats.

[hanna-kruppe]

Oh, right, sorry!

[alexcrichton]

The libs team discussed this during triage yesterday, and we reached a few conclusions

• There... are many overlapping concerns on this PR. It's ended up being unclear that this was even a libs team PR.
• This PR adds support for a target_float #[cfg] directive defined in custom target specs, but it only goes halfway. The language still defines the types and they're still available for use.
• In terms of an API, we likely want to consider this in conjunction with architecture-dependent types. For example AtomicU64 might also be an architecture-dependent type. We've long wanted a mechanism for this, but haven't yet designed one.

For now we concluded that this PR may be a bit premature. There are enough concerns here (and this is touching core-enough types) that this likely wants to go through an RFC before a PR. Thanks regardless though for the PR @Zoxc!