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

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.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D122617

Author: Siva Chandra Reddy

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -389,7 +389,12 @@ function(add_integration_test test_name)
       ${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}