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

fusl/arch/arm/src/__set_thread_area.c

+#include <stdint.h>
+#include <elf.h>
+#include "pthread_impl.h"
+#include "libc.h"
+
+#define HWCAP_TLS (1 << 15)
+
+extern const unsigned char __attribute__((__visibility__("hidden")))
+        __a_barrier_dummy[], __a_barrier_oldkuser[],
+        __a_barrier_v6[], __a_barrier_v7[],
+        __a_cas_dummy[], __a_cas_v6[], __a_cas_v7[],
+        __a_gettp_dummy[];
+
+#define __a_barrier_kuser 0xffff0fa0
+#define __a_cas_kuser 0xffff0fc0
+#define __a_gettp_kuser 0xffff0fe0
+
+extern uintptr_t __attribute__((__visibility__("hidden")))
+        __a_barrier_ptr, __a_cas_ptr, __a_gettp_ptr;
+
+#define SET(op,ver) (__a_##op##_ptr = \
+        (uintptr_t)__a_##op##_##ver - (uintptr_t)__a_##op##_dummy)
+
+int __set_thread_area(void *p)
+{
+#if !__ARM_ARCH_7A__ && !__ARM_ARCH_7R__ && __ARM_ARCH < 7
+        if (__hwcap & HWCAP_TLS) {
+                size_t *aux;
+                SET(cas, v7);
+                SET(barrier, v7);
+                for (aux=libc.auxv; *aux; aux+=2) {
+                        if (*aux != AT_PLATFORM) continue;
+                        const char *s = (void *)aux[1];
+                        if (s[0]!='v' || s[1]!='6' || s[2]-'0'<10u) break;
+                        SET(cas, v6);
+                        SET(barrier, v6);
+                        break;
+                }
+        } else {
+                int ver = *(int *)0xffff0ffc;
+                SET(gettp, kuser);
+                SET(cas, kuser);
+                SET(barrier, kuser);
+                if (ver < 2) a_crash();
+                if (ver < 3) SET(barrier, oldkuser);
+        }
+#endif
+        return __syscall(0xf0005, p);
+}