lib/libc/amd64/string/strchrnul.S: fix edge case in scalar code

When the buffer is immediately preceeded by the character we
are looking for and begins with one higher than that character,
and the buffer is misaligned, a match was errorneously detected
in the first character. Fix this by changing the way we prevent
matches before the buffer from being detected: instead of
removing the corresponding bit from the 0x80..80 mask, set the
LSB of bytes before the buffer after xoring with the character we
look for.

The bug only affects amd64 with ARCHLEVEL=scalar (cf. simd(7)).
The change comes at a 2% performance impact for short strings
if ARCHLEVEL is set to scalar. The default configuration is not
affected.

os: FreeBSD
arch: amd64
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz

│ strchrnul.scalar.0.out │       strchrnul.scalar.2.out       │
│         sec/op         │   sec/op     vs base               │

Short 57.89µ ± 2% 59.08µ ± 1% +2.07% (p=0.030 n=20)
Mid 19.24µ ± 0% 19.73µ ± 0% +2.53% (p=0.000 n=20)
Long 11.03µ ± 0% 11.03µ ± 0% ~ (p=0.547 n=20)
geomean 23.07µ 23.43µ +1.53%

│ strchrnul.scalar.0.out │       strchrnul.scalar.2.out        │
│          B/s           │     B/s       vs base               │

Short 2.011Gi ± 2% 1.970Gi ± 1% -2.02% (p=0.030 n=20)
Mid 6.049Gi ± 0% 5.900Gi ± 0% -2.47% (p=0.000 n=20)
Long 10.56Gi ± 0% 10.56Gi ± 0% ~ (p=0.547 n=20)
geomean 5.045Gi 4.969Gi -1.50%

MFC to: stable/14
MFC after: 3 days
Approved by: mjg (blanket, via IRC)
Sponsored by: The FreeBSD Foundation