[libc++] Replace __ppc64__ with __powerpc64__ and fix is_iec559 for non-ibm128

The lowercase `__ppc64__` is not defined by non-darwin powerpc64 GCC, therefore
it lures users to write code which is not portable to GCC. Migrate to
`__powerpc64__` in preparation for undefining `__ppc64__`. `__powerpc64__` is
much more common than `__PPC64__`.

Update alignment_of.pass.cpp to use 1 unconditionally:
on powerpc-unknown-linux-gnu `alignof(bool) = _Alignof(bool) = __alignof(bool) = 1`.
The value 4 might be derived from an ancient Clang.

Change is_iec559 to true when long double uses uses IEEE 754 quadruple or double
precision (i.e. not ibm128).

Reviewed By: #libc, thesamesam, ldionne

Differential Revision: https://reviews.llvm.org/D137513