[libc] Add ${CMAKE_CROSSCOMPILING_EMULATOR} to custom test cmdlines (#66565)

${CMAKE_CROSSCOMPILING_EMULATOR} will be used in the new rv32 buildbot
and is prepended automatically when we call add_custom_target in CMake,
except when we use a custom command.

There are two places where custom commands are used in libc, so we
explicitly add the ${CMAKE_CROSSCOMPILING_EMULATOR} variable there.
Other systems that don't use ${CMAKE_CROSSCOMPILING_EMULATOR} are
unaffected