Fix minimum-size attribute (currently unused) in gcc

This attribute macro, ORCA_OPT_MINSIZE, isn't currently in use in any of
the code, though it occasionally gets used when testing things. I had
previously only tested this under clang, and hadn't tested it under gcc.
Some of the syntax was missing for gcc's version. This commit fixes it.

base.h

@@ -11,7 +11,7 @@
 #if defined(__clang__)
 #define ORCA_OPT_MINSIZE __attribute__((minsize))
 #elif defined(__GNUC__)
-#define ORCA_OPT_MINSIZE __attribute__(("Os"))
+#define ORCA_OPT_MINSIZE __attribute__((optimize("Os")))
 #else
 #define ORCA_OPT_MINSIZE
 #endif