exec: Split up and tidy code_gen_buffer

It now consists of:

A macro definition of MAX_CODE_GEN_BUFFER_SIZE with host-specific values,

A function size_code_gen_buffer that applies most of the reasoning for
choosing a buffer size,

Three variations of a function alloc_code_gen_buffer that contain all
of the logic for allocating executable memory via a given allocation
mechanism.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>