[kernel][heap] Don't use heap for thread stacks

Kernel stacks are now allocated by vm_allocate_kstack.

Thread creation no longer allows you to bring your own
stack. thread_create and thread_create_etc now always create a stack
of size DEFAULT_STACK_SIZE.

Remove THREAD_STACK_BOUNDS_CHECK feature as it has been subsumed by
guard pages provided by vm_allocate_kstack.

Test: verified no leaks using "k vmm kaspaces" and "k thread_tests"

ZX-2509 #done

Change-Id: I8dbde5c8a6852775b9796e0c7f963289291822b2

[kernel] Rename "in_int_handler" to "blocking_disallowed"

The name "in_int_handler" is misleading because the field is only set
to true for part of the interrupt handler -- the part during which
blocking operations are disallowed.

Rename the field to reflect its actual meaning.

Bug: none
Test: build

Change-Id: Iab41d3650ea805b8de0b13de76b64c5819f48b2f

[kernel] Prefer fbl::count_of in C++ code

This also migrates some C++ code to use fbl::atomic, to help settle
some integer comparison differences.

Change-Id: Ief77949d571351a3990fe9a8251d56d35735422c

[kernel] Move unittest.h to include/lib/unittest/

Move unittest.h deeper in the tree so include statements will say:

Change-Id: I85cb027a53530cd2003d813f004aaea3509f9f40

[kernel][unittest] Remove unused "void* context" argument from test cases

This argument is always NULL.

This argument was part of an init/cleanup feature that was unused and
that was removed.

Change-Id: I997a268be10c6837b2d7529fb92c2df2e3bab814

[kernel][unittest] Remove unused init/cleanup function parameters

None of the in-kernel test cases provide init or cleanup functions.
They all pass NULL to UNITTEST_END_TESTCASE() for those parameters, so
we can remove those parameters. Note that unittest.c wasn't actually
using the cleanup function.

Change-Id: I0332bc5c870deec8a021f543ec2c31ef95c30d0f

[kernel][mp] Set arch_in_int_handler() during sync IPI callbacks

Before this change, sync IPI callbacks were sometimes called with
arch_in_int_handler() set to true (when called from an actual
interrupt) but sometimes called with arch_in_int_handler() set to
false (for two cases in mp_sync_exec()).

For consistency, ensure that arch_in_int_handler() is always set to
true when sync IPI callbacks are called.

This will ensure that if sync IPI callbacks do any blocking
operations, that will always be caught and trigger an assertion
failure (if DEBUG_ASSERT() is enabled).

ZX-1490

Change-Id: Ie4afee5ee43bc3f1d227b5990600d30227184758

[kernel][tests] Convert sync_ipi_tests.cpp to use unittest.h framework

This means that the tests in this file will get run on the bots (on
the CQ, etc.).

ZX-1617

Change-Id: Ic25caeeff2d9754918accc97ea2fae6ce01a532c

[kernel] Remove kernel/app module

This is no longer used at all.
The kernel/app/tests code doesn't use the "app" infrastructure.
So move it to kernel/tests.

Change-Id: I6cd8ec1bf569fd95940e5ebf4dc5f640b265eb69