libaom v3.1.3 release candidate 1

[Wan-Teh Chang]

Hi everyone,

libaom v3.1.3 release candidate 1 is available. The release tag is
v3.1.3-rc1. Your testing will be greatly appreciated, especially on
macOS with the Apple M1 chip or on Windows with MSVC.

We are planning to create the final release on Friday, Oct 1, 2021.

This release includes several bug fixes.

- Bug fixes:
Issue 3147: Build failure on Apple M1 arm64

The following four cmake changes should help the people building
libaom using MSVC.
1. exports: use CMAKE_SHARED_LIBRARY_PREFIX to determine lib name
https://aomedia-review.googlesource.com/c/aom/+/142342
2. aom_install: Install lib dlls to bindir
https://aomedia-review.googlesource.com/c/aom/+/146546
3. aom_install: use relpath for install
https://aomedia-review.googlesource.com/c/aom/+/146550
4. aom_install: don't exclude msvc from install
https://aomedia-review.googlesource.com/c/aom/+/146547

Thanks,
Wan-Teh Chang