Add ARM BX instruction for function returns

The BX (Branch and Exchange) instruction is needed for proper function
returns in ARM code generation. Unlike BLX which saves the return
address, BX simply branches to the address in the register.

This fixes undefined reference errors during compilation.

Author: jserv

src/arm.c

@@ -329,6 +329,12 @@ int __blx(arm_cond_t cond, arm_reg rd)
     return arm_encode(cond, 18, 15, 15, rd + 3888);
 }
 
+int __bx(arm_cond_t cond, arm_reg rm)
+{
+    /* BX: Branch and Exchange */
+    return (cond << 28) | 0x012FFF10 | rm;
+}
+
 int __mul(arm_cond_t cond, arm_reg rd, arm_reg r1, arm_reg r2)
 {
     return arm_encode(cond, 0, rd, 0, (r1 << 8) + 144 + r2);