[libc++] Refactor the selection of string's ABI in __config

This doesn't change the selection, but it expands the conditions to
add comments and make it clearer what's happening. It also removes a
-Wundef instance when we checked __ARM_ARCH_7K__ >= 2 without checking
that it is defined in the first place.

Differential Revision: https://reviews.llvm.org/D153413