updates for GCC 6.4:

- remove many _DIAGASSERT() checks against not NULL for functions
with arguments with nonnull attributes. (probably more to come,
the set between x86 and sparc us disjoint.

updates for GCC 6.4:

- remove many _DIAGASSERT() checks against not NULL for functions
with arguments with nonnull attributes. (probably more to come,
the set between x86 and sparc us disjoint.)

- port libsanitizer's GetPcSpBp() to sparc, sparc64 and amd64.

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libkern.h maps memcmp, memset and strcpy to the compiler builtins on
some platforms, so make sure to #undef them first.

handle fortify, ansify.

Merge libkern + libc common files. As requested by core.