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

fusl/arch/x86_64/bits/user.h

+#undef __WORDSIZE
+#define __WORDSIZE 64
+
+typedef struct user_fpregs_struct
+{
+        uint16_t cwd, swd, ftw, fop;
+        uint64_t rip, rdp;
+        uint32_t mxcsr, mxcr_mask;
+        uint32_t st_space[32], xmm_space[64], padding[24];
+} elf_fpregset_t;
+
+struct user_regs_struct
+{
+        unsigned long r15, r14, r13, r12, rbp, rbx, r11, r10, r9, r8;
+        unsigned long rax, rcx, rdx, rsi, rdi, orig_rax, rip;
+        unsigned long cs, eflags, rsp, ss, fs_base, gs_base, ds, es, fs, gs;
+};
+#define ELF_NGREG 27
+typedef unsigned long long elf_greg_t, elf_gregset_t[ELF_NGREG];
+
+struct user
+{
+        struct user_regs_struct         regs;
+        int                             u_fpvalid;
+        struct user_fpregs_struct       i387;
+        unsigned long                   u_tsize;
+        unsigned long                   u_dsize;
+        unsigned long                   u_ssize;
+        unsigned long                   start_code;
+        unsigned long                   start_stack;
+        long                            signal;
+        int                             reserved;
+        struct user_regs_struct         *u_ar0;
+        struct user_fpregs_struct       *u_fpstate;
+        unsigned long                   magic;
+        char                            u_comm[32];
+        unsigned long                   u_debugreg[8];
+};
+
+#define PAGE_MASK               (~(PAGE_SIZE-1))
+#define NBPG                    PAGE_SIZE
+#define UPAGES                  1
+#define HOST_TEXT_START_ADDR    (u.start_code)
+#define HOST_STACK_END_ADDR     (u.start_stack + u.u_ssize * NBPG)