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

fusl/arch/arm/bits/fenv.h

+#ifndef __ARM_PCS_VFP
+#define FE_ALL_EXCEPT 0
+#define FE_TONEAREST  0
+#else
+#define FE_INVALID    1
+#define FE_DIVBYZERO  2
+#define FE_OVERFLOW   4
+#define FE_UNDERFLOW  8
+#define FE_INEXACT    16
+#define FE_ALL_EXCEPT 31
+#define FE_TONEAREST  0
+#define FE_DOWNWARD   0x800000
+#define FE_UPWARD     0x400000
+#define FE_TOWARDZERO 0xc00000
+#endif
+
+typedef unsigned long fexcept_t;
+
+typedef struct {
+        unsigned long __cw;
+} fenv_t;
+
+#define FE_DFL_ENV      ((const fenv_t *) -1)