[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

Author: mikhailramalho

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -551,6 +551,7 @@ function(add_integration_test test_name)
   set(test_cmd
       ${INTEGRATION_TEST_ENV}
       $<$<BOOL:${LIBC_TARGET_ARCHITECTURE_IS_GPU}>:${gpu_loader_exe}>
+      ${CMAKE_CROSSCOMPILING_EMULATOR}
       ${INTEGRATION_TEST_LOADER_ARGS}
       $<TARGET_FILE:${fq_build_target_name}> ${INTEGRATION_TEST_ARGS})
   add_custom_target(
@@ -711,7 +712,7 @@ function(add_libc_hermetic_test test_name)
   endif()
 
   set(test_cmd ${HERMETIC_TEST_ENV}
-      $<$<BOOL:${LIBC_TARGET_ARCHITECTURE_IS_GPU}>:${gpu_loader_exe}> ${HERMETIC_TEST_LOADER_ARGS}
+      $<$<BOOL:${LIBC_TARGET_ARCHITECTURE_IS_GPU}>:${gpu_loader_exe}> ${CMAKE_CROSSCOMPILING_EMULATOR} ${HERMETIC_TEST_LOADER_ARGS}
       $<TARGET_FILE:${fq_build_target_name}> ${HERMETIC_TEST_ARGS})
   add_custom_target(
     ${fq_target_name}