fix broken exponential asm

infinities were getting converted into nans. the new code simply tests
for infinity and replaces it with a large magnitude value of the same
sign.

also, the fcomi instruction is apparently not part of the i387
instruction set, so avoid using it.

src/math/i386/exp.s

@@ -34,6 +34,15 @@ exp:
 .type exp2,@function
 exp2:
 	fldl 4(%esp)
+1:	fxam
+	fnstsw %ax
+	sahf
+	jnp 1f
+	jnc 1f
+	fstps 4(%esp)
+	mov $0xfe,%al
+	and %al,7(%esp)
+	flds 4(%esp)
 1:	fld %st(0)
 	frndint
 	fxch %st(1)

src/math/i386/expm1.s

@@ -14,12 +14,23 @@ expm1l:
 .type expm1,@function
 expm1:
 	fldl 4(%esp)
+1:	fxam
+	fnstsw %ax
+	sahf
+	jnp 1f
+	jnc 1f
+	fstps 4(%esp)
+	mov $0xfe,%al
+	and %al,7(%esp)
+	flds 4(%esp)
 1:	fldl2e
 	fmulp
 	fld %st(0)
 	frndint
 	fldz
-	fcompi
+	fcomp
+	fnstsw %ax
+	sahf
 	jnz 1f
 	fstp %st(0)
 	f2xm1