[gn] port a6643f2 (libc++ picolib/newlib)

Author: nico

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -40,6 +40,8 @@ if (current_toolchain == default_toolchain) {
       "_LIBCPP_INSTRUMENTED_WITH_ASAN=",
       "_LIBCPP_ABI_DEFINES=",
       "_LIBCPP_HARDENING_MODE_DEFAULT=_LIBCPP_HARDENING_MODE_NONE",
+      "_LIBCPP_LIBC_PICOLIBC=",
+      "_LIBCPP_LIBC_NEWLIB=",
       "_LIBCPP_PSTL_BACKEND_LIBDISPATCH=",
       "_LIBCPP_PSTL_BACKEND_SERIAL=",
       "_LIBCPP_PSTL_BACKEND_STD_THREAD=1",