lib: Prepare zstd for preboot environment, improve performance

These changes are necessary to get the build to work in the preboot
environment, and to get reasonable performance:

- Remove a double definition of the CHECK_F macro when the zstd
  library is amalgamated.

- Switch ZSTD_copy8() to __builtin_memcpy(), because in the preboot
  environment on x86 gcc can't inline `memcpy()` otherwise.

- Limit the gcc hack in ZSTD_wildcopy() to the broken gcc version. See
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81388.

ZSTD_copy8() and ZSTD_wildcopy() are in the core of the zstd hot loop.
So outlining these calls to memcpy(), and having an extra branch are very
detrimental to performance.

Signed-off-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200730190841.2071656-2-nickrterrell@gmail.com

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