Error for multiple `-arch` flags

From [a user list mail](https://lists.boost.org/boost-users/2018/04/88493.php):

> I got this error:
>
> ./b2 toolset=clang 'cxxflags=-arch i386 -arch x86_64'
>
> /Users/gjasny/Git/ExternalLibs/boost/libs/predef/check/../tools/check/predef.jam:46: Unescaped special character in argument $(language)::$(expression)
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/configure.jam:288: in try-find-build
> *** argument error
> * rule log-check-result ( result )
> * called with: ( )
> * missing argument result
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/configure.jam:86:see definition of rule 'log-check-result' being called
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/configure.jam:391: in find-builds-raw
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/configure.jam:450: in configure.find-builds
> /Users/gjasny/Git/ExternalLibs/boost/boostcpp.jam:690: in boostcpp.deduce-address-model
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/kernel/modules.jam:107: in modules.call-in
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/util/indirect.jam:105: in indirect.call
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/property.jam:144: in property.evaluate-conditionals-in-context
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:1087: in evaluate-requirements
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:1121: in common-properties2
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:1017: in targets.common-properties
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:1313: in alias-target-class.generate
> /Users/gjasny/Git/ExternalLibs/boost/boostcpp.jam:500: in build-multiple
> /Users/gjasny/Git/ExternalLibs/boost/boostcpp.jam:490: in class_at_top-level-target.generate
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:812: in generate-really
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:784: in class_at_main-target.generate
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build/targets.jam:273: in class_at_project-target.generate
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/build-system.jam:797: in load
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/kernel/modules.jam:295: in import
> /Users/gjasny/Git/ExternalLibs/boost/tools/build/src/kernel/bootstrap.jam:139: in boost-build
> /Users/gjasny/Git/ExternalLibs/boost/boost-build.jam:17: in module scope
>
> I guess b2 when trying to deduce compiler settings stumbles over
> 'cxxflags=-arch i386 -arch x86_64'. In case that's accepted behavior,
> how could I specify architectures for a fat library?