Some GCC flags improvements (#1903)

* Some GCC flags improvements

* Move -fno-reorder-blocks -fno-zero-initialized-in-bss to assets only

* Add -fno-PIC since some gcc versions use -fPIC as default

* Enable builtin functions on gcc, with appropriate changes to missing_gcc_functions.c
  and ultra64/libc.h

* Move -fno-merge-constants -mno-explicit-relocs -mno-split-addresses to overlays only
  as they are only needed there for reloc reasons

* Remove unneeded casts in missing_gcc_functions.c

* Change gcc assets flags handling

src/gcc_fix/missing_gcc_functions.c

@@ -10,10 +10,10 @@
 // Self-hosted libc memory functions, gcc assumes these exist even in a freestanding
 // environment and there is no way to tell it otherwise.
 
-int memcmp(void* s1, const void* s2, size_t n) {
-    u8* m1 = (u8*)s1;
-    u8* m2 = (u8*)s2;
-    u32 i;
+int memcmp(const void* s1, const void* s2, size_t n) {
+    const u8* m1 = s1;
+    const u8* m2 = s2;
+    size_t i;
 
     for (i = 0; i < n; i++) {
         if (m1[i] < m2[i]) {
@@ -26,17 +26,38 @@ int memcmp(void* s1, const void* s2, size_t n) {
     return 0;
 }
 
-void* memset(void* str, s32 c, size_t n) {
-    u8* m1 = (u8*)str;
-    u32 i;
+void* memset(void* str, int c, size_t n) {
+    u8* m = str;
+    size_t i;
 
     for (i = 0; i < n; i++) {
-        m1[i] = c;
+        m[i] = c;
     }
 
     return str;
 }
 
+void* memmove(void* dest, const void* src, size_t len) {
+    u8* d = dest;
+    const u8* s = src;
+
+    if (d == s) {
+        return dest;
+    }
+    if (d < s) {
+        while (len--) {
+            *d++ = *s++;
+        }
+    } else {
+        d += len - 1;
+        s += len - 1;
+        while (len--) {
+            *d-- = *s--;
+        }
+    }
+    return dest;
+}
+
 // Conversions involving 64-bit integer types required by the O32 MIPS ABI.
 
 // f32 -> u64, negative values become 0