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

fusl/arch/x86_64/bits/fenv.h

+#define FE_INVALID    1
+#define __FE_DENORM   2
+#define FE_DIVBYZERO  4
+#define FE_OVERFLOW   8
+#define FE_UNDERFLOW  16
+#define FE_INEXACT    32
+
+#define FE_ALL_EXCEPT 63
+
+#define FE_TONEAREST  0
+#define FE_DOWNWARD   0x400
+#define FE_UPWARD     0x800
+#define FE_TOWARDZERO 0xc00
+
+typedef unsigned short fexcept_t;
+
+typedef struct {
+        unsigned short __control_word;
+        unsigned short __unused1;
+        unsigned short __status_word;
+        unsigned short __unused2;
+        unsigned short __tags;
+        unsigned short __unused3;
+        unsigned int __eip;
+        unsigned short __cs_selector;
+        unsigned int __opcode:11;
+        unsigned int __unused4:5;
+        unsigned int __data_offset;
+        unsigned short __data_selector;
+        unsigned short __unused5;
+        unsigned int __mxcsr;
+} fenv_t;
+
+#define FE_DFL_ENV      ((const fenv_t *) -1)