Userland: Allow building SerenityOS with -funsigned-char

Some of the code assumed that chars were always signed while that is
not the case on ARM hosts.

Also, some of the code tried to use EOF (-1) in a way similar to what
fgetc() does, however instead of storing the characters in an int
variable a char was used.

While this seemed to work it also meant that character 0xFF would be
incorrectly seen as an end-of-file.

Careful reading of fgetc() reveals that fgetc() stores character
data in an int where valid characters are in the range of 0-255 and
the EOF value is explicitly outside of that range (usually -1).

Userland/Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -40,7 +40,7 @@ static Value parse_simplified_iso8601(const String& iso_8601)
 
     int year = -1, month = -1, day = -1;
     int hours = -1, minutes = -1, seconds = -1, milliseconds = -1;
-    char timezone = -1;
+    int timezone = -1;
     int timezone_hours = -1, timezone_minutes = -1;
     auto lex_year = [&]() {
         if (lexer.consume_specific('+'))