sw::Thread and rr::Thread uses have been replaced by std::thread.

Note the latter does not support controlling the stack size, but it
appears to default to 2 MiB on Linux, 1 MiB on Windows, and 512 KiB on
macOS.

Bug: b/149829034
Change-Id: Ie4ad8af12c111e951761248b69f8206dd143af24
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41368Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>