[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][unittest] Rename REQUIRE_* to ASSERT_* to match userland's unittest.h

This is also more consistent with gtest.

Change-Id: Ic39e75c41a55ca5cba2d09cb920c7cd1c1c7a899

[kernel][libc] Select x86 memcpy/memset implementation at boot

This allows us to make use of ERMS on devices that support it, and
fallback to something more performant otherwise.

ZX-1395 #comment Done for kernel-mode
ZX-1346 #comment This provides runtime selection for memcpy/memset

Change-Id: I96e8ca96e17a677ffa95c59cd36682cb16f2904b