Add a "fork" of musl as //fusl.

fusl/src/internal/vis.h

+/* This file is only used if enabled in the build system, in which case it is
+ * included automatically via command line options. It is not included
+ * explicitly by any source files or other headers. Its purpose is to
+ * override default visibilities to reduce the size and performance costs
+ * of position-independent code. */
+
+#ifndef CRT
+
+/* Conceptually, all symbols should be protected, but some toolchains
+ * fail to support copy relocations for protected data, so exclude all
+ * exported data symbols. */
+
+__attribute__((__visibility__("default")))
+extern struct _IO_FILE *const stdin, *const stdout, *const stderr;
+
+__attribute__((__visibility__("default")))
+extern int optind, opterr, optopt, optreset, __optreset, getdate_err, h_errno, daylight, __daylight, signgam, __signgam;
+
+__attribute__((__visibility__("default")))
+extern long timezone, __timezone;
+
+__attribute__((__visibility__("default")))
+extern char *optarg, **environ, **__environ, *tzname[2], *__tzname[2], *__progname, *__progname_full;
+
+#pragma GCC visibility push(protected)
+
+#endif