[kernel][x64] Use VM stacks for secondary CPU bootstrap threads

Use VM allocated stacks instead of heap allocated stacks for x64
secondary CPU bootstrap threads.

This change eliminates another source of multi-page heap allocations.

Test: Ran "k thread_tests" on x64/gcc, x64/clang. Counted stack vmo's
and vmar's to verify no leaks.

ZX-2547 #comment x64 secondary cpu bootstrap threads

Change-Id: I2bd14f1c7e0858fa63cf67a030b7a892010c24dd

[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

[x86] Run clang-format

This converts a bunch of the old LK style to the newer style.

Change-Id: I0f9dfc6b2b9b30b7630dc1e2271e8da815edd1d8

[x86][cpp] Change NULL -> nullptr

Change-Id: I7e93eece6b0f33712c9c5599a924a44c0ecd4002

[x86][bootstrap16] Enforce exclusive access to bootstrap16

Since both AP startup and resume-from-suspend will need to use this
region, provide a mechanism to ensure they don't stomp on each other.

ZX-1184

Change-Id: I98b0d6a813df2267ca01f945eb57342fdc735b05

[x86][bootstrap16] Allow platform layer to specify the bootstrap region

This is part 1 of moving us to a safer bootstrap region selection. Next
step is snooping the memory map to find a good address.

Change-Id: I1050191c8cf8b1e7a125d74c3bb824c26be4324d

[kernel][x86] Remove arch/x86/64/ subdirectory

Zircon only supports x86-64.
The historical subdirectory split just makes it harder to find all the code.

Change-Id: I7c828660bd1d4d277718aa74f35a3fab5f2333ea