[libc++/libc++abi] Properly delimit lit substitutions

lit is not very clever when it performs substitution on RUN lines. It
simply looks for a match anywhere in the line (without tokenization)
and replaces it by the expansion. This means that a RUN line containing
e.g. `-verify-ignore-unexpected=note` wouod be expanded to
`-verify-ignore-unexpected=<substitution for not>e`, which is
surprising and nonsensical.

It also means that something like `%compile_module` could be expanded
to `<substitution-for-%compile>_module` or to the correct substitution,
depending on the order in which substitutions are evaluated by lit.

To avoid such problems, it is a good habit to delimit custom substitutions
with some token. This commit does that for all substitutions used in the
libc++ and libc++abi test suites.