[Picolibc] TLS bug in ARM build

[Keith Packard]

Debian just got a new version of gcc for arm-none-eabi and I tried
building picolibc with it -- after fixing the test code,
the fcvtbuf test was hanging. I discovered that __aeabi_read_tp was
smashing registers (r3) that it wasn't supposed to. Oops. I ended up
fixing this by re-writing that function in assembly so that it only
modified r0 and lr.

Things work much better now.

If you were using picolibc with an ARM compiler built with TLS support
(which seems to be unusual), you'll want to get this fix.

-- 
-keith
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