Fix Keccak undefined behavior on exotic platforms.

Bug reported and diagnosed in:
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=975977

Fixes bug #40210.
7 jobs for bug40210_035 in 34 minutes and 45 seconds (queued for 34 seconds)
latest
Status Job ID Name Coverage
  Test
passed #7414
debian-disable-dirauth

00:09:16

passed #7411
debian-distcheck

00:12:47

passed #7412
debian-docs

00:08:05

failed #7410
debian-hardened

00:10:48

passed #7413
debian-integration

00:17:06

passed #7409
debian-minimal

00:10:03

passed #7415
debian-nss

00:04:53

 
Name Stage Failure
failed
debian-hardened Test
make[1]: Leaving directory '/builds/asn/tor/build'
make: *** [Makefile:5795: all] Error 2
ERROR: command 'make -j4 -k all' has failed.
Uploading artifacts for failed job
Uploading artifacts...
artifacts/: found 1 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=7410 responseStatus=201 Created token=4xvrtBdN
Cleaning up file based variables
ERROR: Job failed: exit code 1