[libc++] Bump timeout to avoid spurious failures on AIX

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -478,7 +478,7 @@ legacy-project-build)
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
-          -DLLVM_LIT_ARGS="-sv --show-unsupported --xunit-xml-output test-results.xml --timeout=1200" \
+          -DLLVM_LIT_ARGS="-sv --show-unsupported --xunit-xml-output test-results.xml --timeout=1500" \
           -DLIBCXX_CXX_ABI=libcxxabi
     check-runtimes
 ;;