[libc++] tests with picolibc: prevent looking for unneeded "rt" library (#82262)

Picolibc does not provide the clock_gettime function nor the "rt" library.
check_library_exists was invalidly detecting the "rt" library due to cmake issue 
present, when cross-compiling[1]. This resulted with "chrono.cpp" trying to link
to the "rt" library and following error:
    unable to find library from dependent library specifier: rt

[1] https://gitlab.kitware.com/cmake/cmake/-/issues/18121