Speed up tail processing when hashing aligned C strings, take two

After encountering the NUL terminator, the word-at-a-time loop exits
and we must hash the remaining bytes. Previously we calculated
the terminator's position and re-loaded the remaining bytes from
the input string. This was slower than the unaligned case for very
short strings. We already have all the data we need in a register,
so let's just mask off the bytes we need and hash them immediately.

In addition to endianness issues, the previous attempt upset valgrind
in the way it computed the mask. Whether by accident or by wisdom,
the author's proposed method passes locally with valgrind 3.22.

Ants Aasma, with cosmetic adjustments by me

Discussion: https://postgr.es/m/CANwKhkP7pCiW_5fAswLhs71-JKGEz1c1%2BPC0a_w1fwY4iGMqUA%40mail.gmail.com

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