[flang-rt] Use -Wp to undefine macros (#156034)
If the macro was previously defined with `-Wp,-D` then a later `-U` is
*not* going to take effect, the `-Wp` flag takes precedence.
Instead use `-Wp,-U`. This works regardless of whether the original
definition was provided via `-D` or `-Wp,-D`.
Also make sure these flags only get passed to the C++ compiler -- they
are only relevant there, and flang does not support `-Wp`.
