Fix help text for '--no-clang' flag.

Setting the '--no-clang' flag makes the build use gcc/g++ instead of clang, contrary to current help text.

Bug: 238217027
Tag: #floss
Test: Manual - Run "./build.py --no-clang" and see that gcc/g++ is being run instead of clang.

Change-Id: I0f920141b7bda7ff586ba40d4a97dcefd7d2e7f8