Disable default non-portable build for armel

[gbalykov]

This is required for build of native part of tests as non-portable and managed part of tests as portable. Also, this makes armel tizen build consistent with arm64 tizen build scheme, which requires --portable false option. Example of tizen build:

ROOTFS_DIR=`pwd`/.tools/rootfs/armel ./build.sh --portablebuild false --cross --clang9 --arch armel --configuration Release --subset clr+libs.native
ROOTFS_DIR=`pwd`/.tools/rootfs/armel ./build.sh --cross --clang9 --arch armel --configuration Release --subset libs.ref+libs.src+libs.pretest+libs.packages+libs.tests

Build of tests can be performed similar to #40979 (comment):

cd src/coreclr
ROOTFS_DIR=`pwd`/../../.tools/rootfs/armel ./build-test.sh -release portablebuild=false -cross -armel -clang9 -priority1 skipgenerateversion skipstressdependencies skipmanaged skipgeneratelayout skiprestorepackages /p:__SkipPackageRestore=true
ROOTFS_DIR=`pwd`/../../.tools/rootfs/armel ./build-test.sh -release -cross -armel -clang9 -priority1 skipgenerateversion skipstressdependencies skipnative skipgeneratelayout skiptestwrappers
ROOTFS_DIR=`pwd`/../../.tools/rootfs/armel ./build-test.sh -release -cross -armel -clang9 -priority1 skipgenerateversion skipstressdependencies copynativeonly
ROOTFS_DIR=`pwd`/../../.tools/rootfs/armel ./build-test.sh -release -cross -armel -clang9 -priority1 skipgenerateversion skipstressdependencies generatelayoutonly

cc @alpencolt @jkotas

[jkotas]

LGTM

[jkotas]

CI failures are #41006