[REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system

[Kyle Huey]

On the x86-64 5.9-rc1 TLS is completely broken in 32 bit tracees when
running under rr[0]. Booting the kernel with `nofsgsbase` fixes it and
I bisected to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8&id=673903495c85137791d5820d690229efe09c8f7b.

STR:
1. Build rr from source by
  a. git clone https://github.com/mozilla/rr
  b. mkdir rr/obj
  c. cd rr/obj
  d. cmake ..
  e. make -j16
2. Run the simple 32 bit tracee outside of rr with `./bin/simple_32`.
It should print a message and exit cleanly.
3. Run it under rr with `./bin/rr ./bin/simple_32`.

It should behave the same way, but with fsgsbase enabled it will
segfault. The `simple_32` binary is a simple "hello world" type
program but it does link to pthreads, so pre-main code attempts to
access TLS variables.

The interplay between 32 bit and 64 bit TLS is dark magic to me
unfortunately so this is all the useful information I have.

- Kyle

[0] https://rr-project.org/