Do not use `MRB_METHOD_TABLE_INLINE` by default; fix #3924

It was default on Linux. Abandoned for 2 reasons:

* some cross-platform compiler defines `__linux__` even on bare metal
  environment (RTL8196 (MIPS big endian soc) for example).

* some compilers do not align functions pointers so that we need to
  specify `-falign-functions=n` (where n>1). It's not suitable for
  default configuration.

By our measurement, `mrbtest` consumes 400KB less memory. So if your
target machine is memory tight, add the following command-line options
to `GCC` (unfortunately `clang` does not support `-falign-functions`).

`-falign-functions=2 -DMRB_METHOD_TABLE_INLINE`

include/mrbconf.h

@@ -36,12 +36,10 @@
 /* size of the method cache (need to be the power of 2) */
 //#define MRB_METHOD_CACHE_SIZE (1<<7)
 
-/* add -DMRB_METHOD_TABLE_INLINE unless platform uses MSB of pointers */
+/* add -DMRB_METHOD_TABLE_INLINE to reduce the size of method table */
+/* MRB_METHOD_TABLE_INLINE requires LSB of function pointers to be zero */
+/* you might need to specify --falign-functions=n (where n>1) */
 //#define MRB_METHOD_TABLE_INLINE
-/* turn MRB_METHOD_TABLE_INLINE on for linux by default */
-#if !defined(MRB_METHOD_TABLE_INLINE) && defined(__linux__)
-# define MRB_METHOD_TABLE_INLINE
-#endif
 
 /* add -DMRB_INT16 to use 16bit integer for mrb_int; conflict with MRB_INT64 */
 //#define MRB_INT16