swscale: save ebx register when it is not available

Configure checks if the ebx register can be used for asm and it has to
be saved if and only if this is not the case.
Without this the build fails when configuring with --toolchain=hardened
--disable-pic on i386 using gcc 4.8:
error: PIC register clobbered by '%ebx' in 'asm'

In that case gcc 4.8 reserves the ebx register for the GOT needed for
PIE, so it can't be used in asm directly.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libswscale/x86/hscale_fast_bilinear_simd.c

@@ -199,7 +199,7 @@ void ff_hyscale_fast_mmxext(SwsContext *c, int16_t *dst,
 #if ARCH_X86_64
     uint64_t retsave;
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
     uint64_t ebxsave;
 #endif
 #endif
@@ -209,7 +209,7 @@ void ff_hyscale_fast_mmxext(SwsContext *c, int16_t *dst,
         "mov               -8(%%rsp), %%"FF_REG_a"    \n\t"
         "mov            %%"FF_REG_a", %5              \n\t"  // retsave
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
         "mov            %%"FF_REG_b", %5              \n\t"  // ebxsave
 #endif
 #endif
@@ -255,7 +255,7 @@ void ff_hyscale_fast_mmxext(SwsContext *c, int16_t *dst,
         "mov                      %5, %%"FF_REG_a" \n\t"
         "mov            %%"FF_REG_a", -8(%%rsp)    \n\t"
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
         "mov                      %5, %%"FF_REG_b" \n\t"
 #endif
 #endif
@@ -264,12 +264,12 @@ void ff_hyscale_fast_mmxext(SwsContext *c, int16_t *dst,
 #if ARCH_X86_64
           ,"m"(retsave)
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
           ,"m" (ebxsave)
 #endif
 #endif
         : "%"FF_REG_a, "%"FF_REG_c, "%"FF_REG_d, "%"FF_REG_S, "%"FF_REG_D
-#if ARCH_X86_64 || !defined(PIC)
+#if ARCH_X86_64 || HAVE_EBX_AVAILABLE
          ,"%"FF_REG_b
 #endif
     );
@@ -289,7 +289,7 @@ void ff_hcscale_fast_mmxext(SwsContext *c, int16_t *dst1, int16_t *dst2,
 #if ARCH_X86_64
     DECLARE_ALIGNED(8, uint64_t, retsave);
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
     DECLARE_ALIGNED(8, uint64_t, ebxsave);
 #endif
 #endif
@@ -298,7 +298,7 @@ void ff_hcscale_fast_mmxext(SwsContext *c, int16_t *dst1, int16_t *dst2,
         "mov          -8(%%rsp), %%"FF_REG_a"    \n\t"
         "mov       %%"FF_REG_a", %7              \n\t"  // retsave
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
         "mov       %%"FF_REG_b", %7              \n\t"  // ebxsave
 #endif
 #endif
@@ -332,7 +332,7 @@ void ff_hcscale_fast_mmxext(SwsContext *c, int16_t *dst1, int16_t *dst2,
         "mov                    %7, %%"FF_REG_a" \n\t"
         "mov          %%"FF_REG_a", -8(%%rsp)    \n\t"
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
         "mov %7, %%"FF_REG_b"    \n\t"
 #endif
 #endif
@@ -341,12 +341,12 @@ void ff_hcscale_fast_mmxext(SwsContext *c, int16_t *dst1, int16_t *dst2,
 #if ARCH_X86_64
           ,"m"(retsave)
 #else
-#if defined(PIC)
+#if !HAVE_EBX_AVAILABLE
           ,"m" (ebxsave)
 #endif
 #endif
         : "%"FF_REG_a, "%"FF_REG_c, "%"FF_REG_d, "%"FF_REG_S, "%"FF_REG_D
-#if ARCH_X86_64 || !defined(PIC)
+#if ARCH_X86_64 || HAVE_EBX_AVAILABLE
          ,"%"FF_REG_b
 #endif
     );