Fix fc_nyblswap on LLVM (#60)

With a constraint "a", %0 will expand to "A" in the asm, meaning
"ld%0" will become "ldA" without any address.  But since we have
constrained the operand to already be in A, there is no need to add
any load or store instructions anyway, so just remove them.

Also the global variable "swp" is not needed either, so remove that as well
when using LLVM.

src/fcio.c

@@ -191,11 +191,11 @@ void fc_init(
     fc_textcolor(COLOR_GREEN);
 }
 
-static unsigned char swp;
 unsigned char fc_nyblswap(unsigned char in) // oh why?!
 {
-    swp = in;
 #ifdef __CC65__
+    static unsigned char swp;
+    swp = in;
     __asm__("lda %v", swp);
     __asm__("asl  a");
     __asm__("adc  #$80");
@@ -204,21 +204,20 @@ unsigned char fc_nyblswap(unsigned char in) // oh why?!
     __asm__("adc  #$80");
     __asm__("rol  a");
     __asm__("sta %v", swp);
+    return swp;
 #elif defined(__clang__)
-#pragma GCC warning "LLVM assembly needs to be checked in fc_nyblswap()"
-    asm volatile("ld%0\n"
-                 "asl a\n"
+    asm volatile("asl a\n"
                  "adc #$80\n"
                  "rol a\n"
                  "asl a\n"
                  "adc #$80\n"
                  "rol a\n"
-                 "st%0"
-                 : "+a"(swp));
+                 : "+a"(in));
+    return in;
 #else
 #pragma GCC warning "fc_nyblswap() is not implemented for this compiler"
+    return in;
 #endif
-    return swp;
 }
 
 void fc_flash(byte f)