[libc++] Clarify names of ABI forcing macros

Make it clear that these are intended only to force a specific ABI when
the autodetection would give the wrong result by renaming the cmake
options and adding separate forcing macros, as suggested by EricWF in
the post-commit review of r314949 and further discussed on IRC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@314965 91177308-0d34-0410-b5e6-96231b3b80d8