feat: enable `-Wsign-compare`

This enables the `-Wsign-compare` warning everywhere except inside the
`ASSERT_EQ` macro, which will be fixed in later commits.

Change-Id: I485a0e909f23264c662702d996f792e9411ecd2c
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/manifest.c b/src/manifest.c
index 92afa7b..b317fc8 100644
--- a/src/manifest.c
+++ b/src/manifest.c
@@ -1122,7 +1122,7 @@
 	}
 
 	/* GP register is restricted to one of x0 - x3. */
-	if (vm->partition.gp_register_num != -1 &&
+	if (vm->partition.gp_register_num != DEFAULT_BOOT_GP_REGISTER &&
 	    vm->partition.gp_register_num > 3) {
 		dlog_error("GP register number %s: %u\n", error_string,
 			   vm->partition.gp_register_num);