cgen: fix warnings (#5820)

vlib/os/environment.v

@@ -3,7 +3,7 @@
 // that can be found in the LICENSE file.
 module os
 
-fn C.getenv(byteptr) &char
+fn C.getenv(charptr) &char
 // C.GetEnvironmentStringsW & C.FreeEnvironmentStringsW are defined only on windows
 fn C.GetEnvironmentStringsW() &u16
 
@@ -18,7 +18,7 @@ pub fn getenv(key string) string {
 		}
 		return string_from_wide(s)
 	} $else {
-		s := C.getenv(key.str)
+		s := C.getenv(charptr(key.str))
 		if s == voidptr(0) {
 			return ''
 		}
@@ -36,7 +36,7 @@ pub fn setenv(name string, value string, overwrite bool) int {
 		}
 		return -1
 	} $else {
-		return C.setenv(name.str, value.str, overwrite)
+		return C.setenv(charptr(name.str), charptr(value.str), overwrite)
 	}
 }
 
@@ -46,7 +46,7 @@ pub fn unsetenv(name string) int {
 		format := '${name}='
 		return C._putenv(format.str)
 	} $else {
-		return C.unsetenv(name.str)
+		return C.unsetenv(charptr(name.str))
 	}
 }
 
@@ -73,7 +73,7 @@ pub fn environ() map[string]string {
 	} $else {
 		e := &charptr(C.environ)
 		for i := 0; !isnil(e[i]); i++ {
-			eline := cstring_to_vstring(e[i])
+			eline := cstring_to_vstring(byteptr(e[i]))
 			eq_index := eline.index_byte(`=`)
 			if eq_index > 0 {
 				res[eline[0..eq_index]] = eline[eq_index + 1..]