[compiler-rt] Document that builtins is known to work on OpenBSD.

Differential Revision: https://reviews.llvm.org/D109346

Author: fcambus

compiler-rt/www/index.html

@@ -77,7 +77,7 @@ <h2 id="requirements">Platform Support</h2>
    <p><b>builtins</b> is known to work on the following platforms:</p>
    <ul>
    <li>Machine Architectures: i386, X86-64, SPARC64, ARM, PowerPC, PowerPC 64.</li>
-   <li>OS: AuroraUX, DragonFlyBSD, FreeBSD, NetBSD, Linux, Darwin.</li>
+   <li>OS: AuroraUX, DragonFlyBSD, FreeBSD, NetBSD, OpenBSD, Linux, Darwin.</li>
    </ul>
 
    <p>Most sanitizer runtimes are supported only on Linux x86-64. See tool-specific