[mono] Use unsigned char when computing UTF8 string hashes

The C standard does not specify whether `char` is signed or unsigned,
it is implementation defined.

Apparently Android aarch64 makes a different choice than other
platforms (at least macOS arm64 and Windows x64 give different
results).

Mono uses `mono_metadata_str_hash` in the AOT compiler and AOT runtime
to optimize class name lookup.  As a result, classes whose names
include UTF-8 continuation bytes (with the high bit = 1) will hash
differently in the AOT compiler and on the device.

Fixes dotnet#82187
Fixes dotnet#78638

src/mono/mono/eglib/ghashtable.c

@@ -673,7 +673,7 @@ guint
 g_str_hash (gconstpointer v1)
 {
 	guint hash = 0;
-	char *p = (char *) v1;
+	unsigned char *p = (unsigned char *) v1;
 
 	while (*p++)
 		hash = (hash << 5) - (hash + *p);

src/mono/mono/metadata/metadata.c

@@ -5524,7 +5524,8 @@ guint
 mono_metadata_str_hash (gconstpointer v1)
 {
 	/* Same as g_str_hash () in glib */
-	char *p = (char *) v1;
+	/* note: signed/unsigned char matters - we feed UTF-8 to this function, so the high bit will give diferent results if we don't match. */
+	unsigned char *p = (unsigned char *) v1;
 	guint hash = *p;
 
 	while (*p++) {