Whenever the floating-point values are in integer range, we can use the
various FCVT functions with static rounding mode to perform fast
rounding. I took this opportunity to clean up the architecture
differentiation for these functions, which allows us to use the software
rounding implementation for all extreme and unimplemented cases,
including AArch64.
Also adds more round & trunc tests.
Doesn't use them in libc headers so that those don't have to pull in
AK/Platform.h.
AK_COMPILER_GCC is set _only_ for gcc, not for clang too. (__GNUC__ is
defined in clang builds as well.) Using AK_COMPILER_GCC simplifies
things some.
AK_COMPILER_CLANG isn't as much of a win, other than that it's
consistent with AK_COMPILER_GCC.
