cmake: introduce FIBER_STACK_SIZE option

In scope of the commit 82f4b4a3 ("lib/core/fiber: Increase default
stack size") the default value of fiber stack size is increased up to
512 Kb (you can find the reasons in the aforementioned commit message
and in https://github.com/tarantool/tarantool/issues/3418 description).

Some of the tests in test/PUC-Rio-Lua-5.1-test suite in LuaJIT repo
(e.g. some cases with deep recursion in errors.lua or pm.lua) have
already been tweaked according to the limitations mentioned in
https://github.com/tarantool/tarantool/issues/5782, but the crashes
still occurs while running LuaJIT tests with ASan support enabled.

To make the testing routine more convenient, FIBER_STACK_SIZE option is
introduced to Tarantool CMake machinery. One can provide the size either
by raw digits (i.e. in bytes) or using Kb/Mb suffixes for convenience.

A couple of important nits:
* If the given value is not a multiple of 4Kb, CMake machinery adjusts
  it up to the nearest one greater than this value.
* If the adjusted value is less than 512Kb, configuration fails with the
  corresponding CMake fatal error.

Follows up #3418
Relates to #5782

@TarantoolBot document
Title: introduce FIBER_STACK_SIZE configuration option

To make managing of the default fiber stack size more convenient, the
corresponding CMake option is added.

**NB**: The stack size can't be less than 512Kb and if the given value
is not a multiple of 4Kb, CMake machinery adjusts it up to the nearest
one greater than this value.

(cherry picked from commit ff57f990)