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

fusl/arch/arm/pthread_arch.h

+#if ((__ARM_ARCH_6K__ || __ARM_ARCH_6ZK__) && !__thumb__) \
+ || __ARM_ARCH_7A__ || __ARM_ARCH_7R__ || __ARM_ARCH >= 7
+
+static inline pthread_t __pthread_self()
+{
+        char *p;
+        __asm__ __volatile__ ( "mrc p15,0,%0,c13,c0,3" : "=r"(p) );
+        return (void *)(p+8-sizeof(struct pthread));
+}
+
+#else
+
+static inline pthread_t __pthread_self()
+{
+#ifdef __clang__
+        char *p;
+        __asm__ __volatile__ ( "bl __a_gettp\n\tmov %0,r0" : "=r"(p) : : "cc", "r0", "lr" );
+#else
+        register char *p __asm__("r0");
+        __asm__ __volatile__ ( "bl __a_gettp" : "=r"(p) : : "cc", "lr" );
+#endif
+        return (void *)(p+8-sizeof(struct pthread));
+}
+
+#endif
+
+#define TLS_ABOVE_TP
+#define TP_ADJ(p) ((char *)(p) + sizeof(struct pthread) - 8)
+
+#define MC_PC arm_pc