Fix illumos-x64 build

[am11]

Tested with:

$ docker run --rm -v$(pwd):/runtime -e ROOTFS_DIR=/crossrootfs/x64 \
    mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-illumos-20220531132048-f13d79e \
     /runtime/build.sh -c Release -os illumos -cross -gcc

$ echo $?
0

[am11]

cc @jkotas, @kunalspathak, fixed recent build regreession. __tls_get_addr needed @plt suffix (via EXTERNAL_C_FUNC) for solaris linker to look in linkage table (for relative address resolution) and __has_builtin definition missing in a few places, which I've moved to a common place in minipal.

[jkotas]

Thanks