Fix ARM backend exit code handling

Programs were returning exit code 1 instead of actual return values due
to incorrect use of BLX instruction instead of BX for function returns.

The issue affected both regular function returns and syscall returns:
- OP_return was using blx (branch with link exchange) which incorrectly
  modified the link register, corrupting return values
- __syscall was using direct PC manipulation instead of proper BX
  instruction

Add const qualifiers to read-only function param

This improves const-correctness by adding const qualifiers to function
parameters that are never modified.

Author: jserv

src/arm-codegen.c

@@ -117,7 +117,7 @@ void update_elf_offset(ph2_ir_t *ph2_ir)
             elf_offset += 8;
         return;
     case OP_return:
-        elf_offset += 24;
+        elf_offset += 24; /* mov to r0, stack adjustment, lr load, and return */
         return;
     case OP_trunc:
         elf_offset += 4;
@@ -311,7 +311,7 @@ void emit_ph2_ir(ph2_ir_t *ph2_ir)
         emit(__movt(__AL, __r8, ph2_ir->src1 + 4));
         emit(__add_r(__AL, __sp, __sp, __r8));
         emit(__lw(__AL, __lr, __sp, -4));
-        emit(__blx(__AL, __lr));
+        emit(__bx(__AL, __lr));
         return;
     case OP_add:
         emit(__add_r(__AL, rd, rn, rm));
@@ -480,7 +480,7 @@ void code_generate(void)
     emit(__mov_r(__AL, __r4, __r5));
     emit(__mov_r(__AL, __r5, __r6));
     emit(__svc());
-    emit(__mov_r(__AL, __pc, __lr));
+    emit(__bx(__AL, __lr));
 
     ph2_ir_t *ph2_ir;
     for (ph2_ir = GLOBAL_FUNC->bbs->ph2_ir_list.head; ph2_ir;

src/arm.c

@@ -324,6 +324,11 @@ int __bl(arm_cond_t cond, int ofs)
     return arm_encode(cond, 176, 0, 0, 0) + (o & 16777215);
 }
 
+int __bx(arm_cond_t cond, arm_reg rd)
+{
+    return arm_encode(cond, 18, 15, 15, rd + 3872);
+}
+
 int __blx(arm_cond_t cond, arm_reg rd)
 {
     return arm_encode(cond, 18, 15, 15, rd + 3888);

src/lexer.c

@@ -186,7 +186,7 @@ int hex_digit_value(char c)
     return -1;
 }
 
-bool is_numeric(char buffer[])
+bool is_numeric(const char buffer[])
 {
     bool hex = false;
     int size = strlen(buffer);

src/parser.c

@@ -308,7 +308,7 @@ var_t *resize_var(block_t *block, basic_block_t **bb, var_t *from, var_t *to)
     return from;
 }
 
-int read_numeric_constant(char buffer[])
+int read_numeric_constant(const char buffer[])
 {
     int i = 0;
     int value = 0;

src/ssa.c

@@ -1162,7 +1162,7 @@ void bb_dump(FILE *fd, func_t *func, basic_block_t *bb)
             bb_dump_connection(fd, bb->prev[i].bb, bb, bb->prev[i].type);
 }
 
-void dump_cfg(char name[])
+void dump_cfg(const char name[])
 {
     FILE *fd = fopen(name, "w");
 
@@ -1190,7 +1190,7 @@ void dom_dump(FILE *fd, basic_block_t *bb)
     }
 }
 
-void dump_dom(char name[])
+void dump_dom(const char name[])
 {
     FILE *fd = fopen(name, "w");