Implement a fallback to getting the file size via vFile:stat packet
when the remote server does not implement vFile:size. This makes it
possible to query file sizes from remote gdbserver.
Note that unlike vFile:size, the fallback will not work if the server is
unable to open the file.
While at it, add a few tests for the 'platform get-size' command.
Differential Revision: https://reviews.llvm.org/D107780
21e2d7ce43