RCG DEMOD: Protect %rax from getting clobbered in sincos assembly

Some code in the demod rotation calcs was compiled with the assumption that %rax was preserved across a call to sincos.
This is not the case.  Now sincos saves %rax on the stack.

src/fe/demod/Demodulation.c

@@ -87,6 +87,7 @@ void avx_rotation_struct_set_frequency(avx_rotation_struct* self, double freq, u
 	if (idx >= MAX_STRIDE) return;
 	self->freqhist[idx] = freq;
 	sincos(freq * self->two_pi_over_rate, self->anglexy + 2 * idx + 1, self->anglexy + 2 * idx + 0);
+        RTSLOG_DEBUG("sin=%d cos=%d\n", (int)(1000000.0*self->anglexy[2*idx+1]), (int)(1000000.0*self->anglexy[2*idx]));
 }
 
 

src/include/util/inlineMath_x86_asm.h

@@ -39,7 +39,8 @@ sincos( double __x, double* __sinx, double* __cosx )
 {
     register long double __cosr;
     register long double __sinr;
-    __asm __volatile__( "fsincos\n\t"
+    __asm __volatile__( "push      %%rax\n\t"
+                        "fsincos\n\t"
                         "fnstsw    %%ax\n\t"
                         "testl     $0x400, %%eax\n\t"
                         "jz        1f\n\t"
@@ -52,7 +53,7 @@ sincos( double __x, double* __sinx, double* __cosx )
                         "jnz       2b\n\t"
                         "fstp      %%st(1)\n\t"
                         "fsincos\n\t"
-                        "1:"
+                        "1: pop    %%rax\n\t"
                         : "=t"( __cosr ), "=u"( __sinr )
                         : "0"( __x ) );
     *__sinx = __sinr;