thumbify and add END

Use RET and RETc for returning.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

NetBSD/acorn26 has been using APCS-32 for years, so unifdef -U__APCS_26__.

Implement pthread_atfork() (in libc, because the required threadlib
stub behavior is exactly the same as the usual behavior).

Add some explanatory comments.

Create shared ARM source for libc, and move arm26 port over to using it.
The code is (almost) a direct copy of the current arm26 sources.
It's identical to the current arm32 sources, with the followi

Create shared ARM source for libc, and move arm26 port over to using it.
The code is (almost) a direct copy of the current arm26 sources.
It's identical to the current arm32 sources, with the following exceptions:
- _C_LABEL is used on references to C labels from assembler.
- Function returns in assembler have APCS-26 versions in #ifdef __APCS_26__.
- It uses SoftFloat 2a rather than SoftFloat 1a.

The first two of these should be inconsequential. I believe that SoftFloat 2a
should work on arm32 and be backward-compatible with existing code, but this
is not obviously true. For now, arm32 remains using its own bits of libc.

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