lib/string.c: implement a basic bcmp

A recent optimization in Clang (r355672) lowers comparisons of the
return value of memcmp against zero to comparisons of the return value
of bcmp against zero.  This helps some platforms that implement bcmp
more efficiently than memcmp.  glibc simply aliases bcmp to memcmp, but
an optimized implementation is in the works.

This results in linkage failures for all targets with Clang due to the
undefined symbol.  For now, just implement bcmp as a tailcail to memcmp
to unbreak the build.  This routine can be further optimized in the
future.

Other ideas discussed:

 * A weak alias was discussed, but breaks for architectures that define
   their own implementations of memcmp since aliases to declarations are
   not permitted (only definitions). Arch-specific memcmp
   implementations typically declare memcmp in C headers, but implement
   them in assembly.

 * -ffreestanding also is used sporadically throughout the kernel.

 * -fno-builtin-bcmp doesn't work when doing LTO.

Link: https://bugs.llvm.org/show_bug.cgi?id=41035
Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp
Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13
Link: https://github.com/ClangBuiltLinux/linux/issues/416
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Nick Desaulniers <[email protected]>
Reported-by: Nathan Chancellor <[email protected]>
Reported-by: Adhemerval Zanella <[email protected]>
Suggested-by: Arnd Bergmann <[email protected]>
Suggested-by: James Y Knight <[email protected]>
Suggested-by: Masahiro Yamada <[email protected]>
Suggested-by: Nathan Chancellor <[email protected]>
Suggested-by: Rasmus Villemoes <[email protected]>
Acked-by: Steven Rostedt (VMware) <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Masahiro Yamada <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Cc: David Laight <[email protected]>
Cc: Rasmus Villemoes <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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