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`