[libc] Set rtlib to compiler-rt in integration tests.

[sivachandra]

Summary

The clang driver to picks up compiler runtime files using full paths.
Without this, at least for aarch64, the driver tries to pick up the
compiler runtime files from the working directory.

[michaelrj]

LGTM with nit

libc/cmake/modules/LLVMLibCTestRules.cmake

394

the

[sivachandra]

Fix typo.

libc/cmake/modules/LLVMLibCTestRules.cmake

set_target_properties(${fq_target_name}
       PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
   target_include_directories(
     ${fq_target_name}
     PRIVATE
       ${LIBC_SOURCE_DIR}
       ${LIBC_BUILD_DIR}
       ${LIBC_BUILD_DIR}/include
   )
-  target_link_options(${fq_target_name} PRIVATE --sysroot=${sysroot} -static -stdlib=libc++)
+  # We set a number of link options to prevent picking up system libc binaries.
+  # Also, we restrict the integration tests to fully static executables. The
+  # rtlib is set to compiler-rt to make the compiler drivers pick up the compiler
+  # runtime binaries using full paths. Otherwise, files like crtbegin.o are passed
+  # as is (and not as paths like /usr/lib/.../crtbegin.o).
+  target_link_options(${fq_target_name} PRIVATE --sysroot=${sysroot} -static -stdlib=libc++ --rtlib=compiler-rt)
   add_dependencies(${fq_target_name}
                    ${fq_target_name}.__copy_loader__
                    ${fq_libc_target_name}
                    libc.utils.IntegrationTest.test)
 
   add_custom_command(
     TARGET ${fq_target_name}
     POST_BUILD
     COMMAND ${INTEGRATION_TEST_ENV} $<TARGET_FILE:${fq_target_name}> ${INTEGRATION_TEST_ARGS}
   )
 
   add_dependencies(${INTEGRATION_TEST_SUITE} ${fq_target_name})
 endfunction(add_integration_test)