x86/x64: Fix math.ceil(-0.9) result sign.

Reported by minoki.

src/vm_x86.dasc

@@ -415,9 +415,6 @@
 |.macro sseconst_1, reg, tmp		// Synthesize 1.0.
 |  sseconst_hi reg, tmp, 3ff00000
 |.endmacro
-|.macro sseconst_m1, reg, tmp		// Synthesize -1.0.
-|  sseconst_hi reg, tmp, bff00000
-|.endmacro
 |.macro sseconst_2p52, reg, tmp		// Synthesize 2^52.
 |  sseconst_hi reg, tmp, 43300000
 |.endmacro
@@ -3114,15 +3111,17 @@ static void build_subroutines(BuildCtx *ctx)
   |  addsd xmm1, xmm3			// (|x| + 2^52) - 2^52
   |  subsd xmm1, xmm3
   |  orpd xmm1, xmm2			// Merge sign bit back in.
+  |  sseconst_1 xmm3, RDa
   |  .if mode == 1		// ceil(x)?
-  |    sseconst_m1 xmm2, RDa		// Must subtract -1 to preserve -0.
   |    cmpsd xmm0, xmm1, 6		// x > result?
+  |    andpd xmm0, xmm3
+  |    addsd xmm1, xmm0			// If yes, add 1.
+  |    orpd xmm1, xmm2			// Merge sign bit back in (again).
   |  .else			// floor(x)?
-  |    sseconst_1 xmm2, RDa
   |    cmpsd xmm0, xmm1, 1		// x < result?
+  |    andpd xmm0, xmm3
+  |    subsd xmm1, xmm0			// If yes, subtract 1.
   |  .endif
-  |  andpd xmm0, xmm2
-  |  subsd xmm1, xmm0			// If yes, subtract +-1.
   |.endif
   |  movaps xmm0, xmm1
   |1: