exec: Allocate code_gen_prologue from code_gen_buffer

We had a hack for arm and sparc, allocating code_gen_prologue to a
special section.  Which, honestly does no good under certain cases.
We've already got limits on code_gen_buffer_size to ensure that all
TBs can use direct branches between themselves; reuse this limit to
ensure the prologue is also reachable.

As a bonus, we get to avoid marking a page of the main executable's
data segment as executable.

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