Merge #736

736: revise TLS handling r=stlankes a=stlankes

- ARM used variant 1 of the TLS handling (see https://uclibc.org/docs/tls.pdf

)
- the previous version doesn't work correctly
- using an unsized struct to implement TLS like a C kernel

Co-authored-by: Stefan Lankes <slankes@eonerc.rwth-aachen.de>