MFC r302362,r302363,r302364,r302365,r302373:

r302362:

Fix gcc warnings

• Remove -Wunused-but-set-variable (newcpu)
• Always return VMEXIT_CONTINUE as the code always set retval to that value.

r302363:

Fix gcc warnings

Put cfl/prdt under AHCI_DEBUG #defines as they are only used in
those cases.

r302364:

Fix gcc warnings

Add WRAPPED_CTASSERT macro by annotating CTASSERTs with __unused
to deal with -Wunused-local-typedefs warnings from gcc 4.8+.
All other compilers (clang, etc) use CTASSERT as-is. A more generic
solution for this issue will be proposed after ^/stable/11 is forked.

Consolidate all CTASSERTs under one block instead of inlining them in
functions.

r302365:

Fix gcc warnings

Remove -Wunused-but-set-variable (error). Cast calls with
(void) to note that the return value is explicitly ignored.

r302373:

Fix CTASSERT issue in a more clean way

• Replace all CTASSERT macro instances with static_assert's.
• Remove the WRAPPED_CTASSERT macro; it's now an unnecessary obfuscation.
• Localize all static_assert's to the structures being tested.
• Sort some headers per-style(9).