Make a sed script in doc/arm/Makefile.in portable

BSD sed does not recognize \s as a whitespace matching token.  Make the
sed script in doc/arm/Makefile.in which ensures GitLab identifiers are
not split across lines portable by replacing \s with [[:space:]].
75 jobs for !3058 with michal/make-a-sed-script-in-doc-arm-makefile.in-portable in 12 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Precheck
canceled #672944
amd64 linux
🐞:sid:amd64

00:00:11

canceled #672942
amd64 linux
autoreconf:sid:amd64

00:00:12

canceled #672943
amd64 linux
misc:sid:amd64

00:00:11

 
  Build
canceled #672959
amd64 linux
asan:sid:amd64

canceled #672966
amd64 freebsd
clang:freebsd11.3:amd64

canceled #672967
amd64 freebsd
clang:freebsd12.0:amd64

canceled #672968
amd64 libvirt
clang:openbsd6.6:amd64

canceled #672963
amd64 linux
clang:stretch:amd64

canceled #672964
i386 linux
clang:stretch:i386

canceled #672945
amd64 linux
gcc:alpine3.11:amd64

canceled #672958
amd64 linux
gcc:bionic:amd64

canceled #672951
amd64 linux
gcc:buster:amd64

canceled #672946
amd64 linux
gcc:centos6:amd64

canceled #672947
amd64 linux
gcc:centos7:amd64

canceled #672948
amd64 linux
gcc:centos8:amd64

canceled #672956
amd64 linux
gcc:fedora31:amd64

canceled #672949
amd64 linux
gcc:jessie:amd64

canceled #672952
amd64 linux
gcc:sid:amd64

canceled #672954
i386 linux
gcc:sid:i386

canceled #672950
amd64 linux
gcc:stretch:amd64

canceled #672955
amd64 linux
gcc:tumbleweed:amd64

canceled #672957
amd64 linux
gcc:xenial:amd64

canceled #672970
amd64 windows
msvc:windows:amd64

canceled #672962
amd64 linux
mutexatomics:sid:amd64

canceled #672969
amd64 linux
nolibtool:sid:amd64

canceled #672953
amd64 linux
oot:sid:amd64

canceled #672965
amd64 linux
pkcs11:sid:amd64

canceled #672961
amd64 linux
rwlock:sid:amd64

canceled #672960
amd64 linux
tsan:buster:amd64

 
  Unit
canceled #672984
amd64 linux
unit:asan:sid:amd64

canceled #672990
amd64 freebsd
unit:clang:freebsd11.3:amd64

canceled #672991
amd64 freebsd
unit:clang:freebsd12.0:amd64

canceled #672988
amd64 linux
unit:clang:stretch:amd64

canceled #672971
amd64 linux
unit:gcc:alpine3.11:amd64

canceled #672983
amd64 linux
unit:gcc:bionic:amd64

canceled #672977
amd64 linux
unit:gcc:buster:amd64

canceled #672972
amd64 linux
unit:gcc:centos6:amd64

canceled #672973
amd64 linux
unit:gcc:centos7:amd64

canceled #672974
amd64 linux
unit:gcc:centos8:amd64

canceled #672981
amd64 linux
unit:gcc:fedora31:amd64

canceled #672975
amd64 linux
unit:gcc:jessie:amd64

canceled #672978
amd64 linux
unit:gcc:sid:amd64

canceled #672979
i386 linux
unit:gcc:sid:i386

canceled #672976
amd64 linux
unit:gcc:stretch:amd64

canceled #672980
amd64 linux
unit:gcc:tumbleweed:amd64

canceled #672982
amd64 linux
unit:gcc:xenial:amd64

canceled #672987
amd64 linux allowed to fail
unit:mutexatomics:sid:amd64

canceled #672992
amd64 linux
unit:nolibtool:sid:amd64

canceled #672989
amd64 linux
unit:pkcs11:sid:amd64

canceled #672986
amd64 linux
unit:rwlock:sid:amd64

canceled #672985
amd64 linux allowed to fail
unit:tsan:buster:amd64

 
  System
canceled #673006
amd64 linux
system:asan:sid:amd64

canceled #673011
amd64 freebsd
system:clang:freebsd11.3:amd64

canceled #673012
amd64 freebsd
system:clang:freebsd12.0:amd64

canceled #672993
amd64 linux
system:gcc:alpine3.11:amd64

canceled #673005
amd64 linux
system:gcc:bionic:amd64

canceled #672999
amd64 linux
system:gcc:buster:amd64

canceled #672994
amd64 linux
system:gcc:centos6:amd64

canceled #672995
amd64 linux
system:gcc:centos7:amd64

canceled #672996
amd64 linux
system:gcc:centos8:amd64

canceled #673003
amd64 linux
system:gcc:fedora31:amd64

canceled #672997
amd64 linux
system:gcc:jessie:amd64

canceled #673000
amd64 linux
system:gcc:sid:amd64

canceled #673001
i386 linux
system:gcc:sid:i386

canceled #672998
amd64 linux
system:gcc:stretch:amd64

canceled #673002
amd64 linux
system:gcc:tumbleweed:amd64

canceled #673004
amd64 linux
system:gcc:xenial:amd64

canceled #673009
amd64 linux allowed to fail
system:mutexatomics:sid:amd64

canceled #673013
amd64 linux
system:nolibtool:sid:amd64

canceled #673010
amd64 linux
system:pkcs11:sid:amd64

canceled #673008
amd64 linux
system:rwlock:sid:amd64

canceled #673007
amd64 linux allowed to fail
system:tsan:buster:amd64

 
  Docs
canceled #673014
amd64 linux
docs:sid:amd64

 
  Postcheck
canceled #673016
amd64 linux
cppcheck:gcc:sid:amd64

canceled #673015
amd64 linux
scan-build:buster:amd64