Add compilation-related constants

`ODIN_VERSION_HASH` is the `git` SHA hash of the commit the Odin
compiler was built with.

`ODIN_MICROARCH_STRING` is the string passed to `-microarch` when
the program was built.

`ODIN_OPTIMIZATION_MODE` is an enum value of which optimization mode was
used to build the program.

base/runtime/core.odin

@@ -560,6 +560,19 @@ Odin_Platform_Subtarget_Type :: type_of(ODIN_PLATFORM_SUBTARGET)
 */
 Odin_Sanitizer_Flags :: type_of(ODIN_SANITIZER_FLAGS)
 
+/*
+	// Defined internally by the compiler
+	Odin_Optimization_Mode :: enum int {
+		None       = -1,
+		Minimal    =  0,
+		Size       =  1,
+		Speed      =  2,
+		Aggressive =  3,
+	}
+
+	ODIN_OPTIMIZATION_MODE // is a constant
+*/
+Odin_Optimization_Mode :: type_of(ODIN_OPTIMIZATION_MODE)
 
 /////////////////////////////
 // Init Startup Procedures //