unix: Add asm statements needed to read registers with clang. Code ge…

…nerated by gcc 4.9.0 is unchanged (same statements, different order).

Both are inefficient, saving unmodified registers on the stack.

unix/gccollect.c

@@ -43,11 +43,17 @@ typedef machine_uint_t regs_t[6];
 
 void gc_helper_get_regs(regs_t arr) {
     register long rbx asm ("rbx");
+    asm("" : "=r"(rbx));
     register long rbp asm ("rbp");
+    asm("" : "=r"(rbp));
     register long r12 asm ("r12");
+    asm("" : "=r"(r12));
     register long r13 asm ("r13");
+    asm("" : "=r"(r13));
     register long r14 asm ("r14");
+    asm("" : "=r"(r14));
     register long r15 asm ("r15");
+    asm("" : "=r"(r15));
     arr[0] = rbx;
     arr[1] = rbp;
     arr[2] = r12;