encoding.iconv: Fix iconv type cstrict, add LOCAL encoding

[kbkpbot]

fix clang -cstrict error
add LOCAL encoding: this is ANSI under Windows, UTF-8 under Linux

/tmp/v_1000/xx.01J980Z12CW3YET5B6W4SNBXKN.tmp.c:42438:18: error: passing 'u8 *' (aka 'unsigned char *') to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Werror,-Wpointer-sign]
 42438 |         cd = iconv_open(dst_encoding.str, src_encoding.str);
       |                         ^~~~~~~~~~~~~~~~
/usr/include/iconv.h:43:40: note: passing argument to parameter '__tocode' here
   43 | extern iconv_t iconv_open (const char *__tocode, const char *__fromcode)
      |                                        ^
/tmp/v_1000/xx.01J980Z12CW3YET5B6W4SNBXKN.tmp.c:42438:36: error: passing 'u8 *' (aka 'unsigned char *') to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Werror,-Wpointer-sign]
 42438 |         cd = iconv_open(dst_encoding.str, src_encoding.str);
       |                                           ^~~~~~~~~~~~~~~~
/usr/include/iconv.h:43:62: note: passing argument to parameter '__fromcode' here
   43 | extern iconv_t iconv_open (const char *__tocode, const char *__fromcode)
      |                                                              ^
/tmp/v_1000/xx.01J980Z12CW3YET5B6W4SNBXKN.tmp.c:42448:24: error: incompatible pointer types passing 'u8 **' (aka 'unsigned char **') to parameter of type 'char **' [-Werror,-Wincompatible-pointer-types]
 42448 |         usize res = iconv(cd, &src_ptr, &src_left, &dst_ptr, &dst_left);
       |                               ^~~~~~~~
/usr/include/iconv.h:49:54: note: passing argument to parameter '__inbuf' here
   49 | extern size_t iconv (iconv_t __cd, char **__restrict __inbuf,
      |                                                      ^
/tmp/v_1000/xx.01J980Z12CW3YET5B6W4SNBXKN.tmp.c:42448:45: error: incompatible pointer types passing 'u8 **' (aka 'unsigned char **') to parameter of type 'char **' [-Werror,-Wincompatible-pointer-types]
 42448 |         usize res = iconv(cd, &src_ptr, &src_left, &dst_ptr, &dst_left);
       |                                                    ^~~~~~~~
/usr/include/iconv.h:51:26: note: passing argument to parameter '__outbuf' here
   51 |                      char **__restrict __outbuf,
      |                                        ^
4 errors generated.
builder error:
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .```

[spytheman]

Excellent work.