lib/string: optimized memset

lib/string: optimized memset

The generic memset is defined as a byte at time write.  This is always
safe, but it's slower than a 4 byte or even 8 byte write.

Write a generic memset which fills the data one byte at time until the
destination is aligned, then fills using the largest size allowed, and
finally fills the remaining data one byte at time.

On a RISC-V machine the speed goes from 140 Mb/s to 241 Mb/s, and this the
binary size increase according to bloat-o-meter:

Function     old     new   delta
memset        32     148    +116

Link: https://lkml.kernel.org/r/20210702123153.14093-4-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Laight <David.Laight@aculab.com>
Cc: Drew Fustini <drew@beagleboard.org>
Cc: Emil Renner Berthing <kernel@esmil.dk>
Cc: Guo Ren <guoren@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nick Kossifidis <mick@ics.forth.gr>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>