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

fusl/src/fenv/aarch64/fenv.s

Index: fusl/src/fenv/aarch64/fenv.s
diff --git a/fusl/src/fenv/aarch64/fenv.s b/fusl/src/fenv/aarch64/fenv.s
new file mode 100644
index 0000000000000000000000000000000000000000..f0db5d9d1bea3ba0fbb7ccbe9c2dd22b62084741
--- /dev/null
+++ b/fusl/src/fenv/aarch64/fenv.s
@@ -0,0 +1,67 @@
+.global fegetround
+.type fegetround,%function
+fegetround:
+	mrs x0, fpcr
+	and w0, w0, #0xc00000
+	ret
+
+.global __fesetround
+.type __fesetround,%function
+__fesetround:
+	mrs x1, fpcr
+	bic w1, w1, #0xc00000
+	orr w1, w1, w0
+	msr fpcr, x1
+	mov w0, #0
+	ret
+
+.global fetestexcept
+.type fetestexcept,%function
+fetestexcept:
+	and w0, w0, #0x1f
+	mrs x1, fpsr
+	and w0, w0, w1
+	ret
+
+.global feclearexcept
+.type feclearexcept,%function
+feclearexcept:
+	and w0, w0, #0x1f
+	mrs x1, fpsr
+	bic w1, w1, w0
+	msr fpsr, x1
+	mov w0, #0
+	ret
+
+.global feraiseexcept
+.type feraiseexcept,%function
+feraiseexcept:
+	and w0, w0, #0x1f
+	mrs x1, fpsr
+	orr w1, w1, w0
+	msr fpsr, x1
+	mov w0, #0
+	ret
+
+.global fegetenv
+.type fegetenv,%function
+fegetenv:
+	mrs x1, fpcr
+	mrs x2, fpsr
+	stp w1, w2, [x0]
+	mov w0, #0
+	ret
+
+// TODO preserve some bits
+.global fesetenv
+.type fesetenv,%function
+fesetenv:
+	mov x1, #0
+	mov x2, #0
+	cmn x0, #1
+	b.eq 1f
+	ldp w1, w2, [x0]
+1:	msr fpcr, x1
+	msr fpsr, x2
+	mov w0, #0
+	ret