[kernel] Refactor thread_t's stack fields and vm_allocate_kstack

This is a refactoring change in preparation for moving the remaining
custom kernel stack to use vm_allocate_kstack.

Refactor thread_t's stack fields into a new struct, kstack_t.

Refactor vm_allocate_kstack to map both safe and unsafe stacks.

Test: Ran "k thread_tests" on x64/gcc, x64/clang, arm64/gcc,
arm64/clang. Also, counted stack vmo's and vmar's before and after
tests to verify no leaks.

ZX-2547 #comment refactor

Change-Id: Ia574f825995e125d390a53a25d3c46bbae1e1d7e

[kernel][arm64] clang format arm64 code

Change-Id: Id4b2123b16549c555db28bc878dde434f4dc5506

[kernel][debug] add command to print the backtrace of a blocked thread

Also refactored the thread_print_backtrace routines to be a little
clearer.

Change-Id: I8c87061c9dcee57599d62f254d7680cca6c4e02f

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[kernel] Add comments about the context_switch_frame structs

Add a comment to connect the function x86_64_context_switch() to
struct x86_64_context_switch_frame.

Otherwise, it's not obvious that arch_thread_initialize() populates a
data structure that x86_64_context_switch() later uses.

Do the same for the arm64 equivalents.

Change-Id: Ib58701e131376a24568ca268aae6f72dff8463fb

[kernel][arm64] Create a per cpu structure and use the x18 register to point to it

At the moment use it to store the oft-used current cpu number and
is-in-interrupt flag.

Change-Id: I3203358ca47a3b193ef7cb01356b67b50c74ff0f

[kernel] remove the WITH_SMP flag

Always build the kernel with full SMP support.

Change-Id: I105f8e4127a93113c795ae80e1dc4d4b0d43abe8

[kernel][arm64] mass rename arm64 files from .c to .cpp

No attempt to make them compile

Change-Id: If9eef1aded99e20b3046a387cdb29ef1dbe2e587