Use fstat to access the current file size of a FilePlugin file on unices.

This is much faster than the old seek method. e.g. on a 2021 MBP with an SSD
running Sequoia 15.6.1

	| f fid |
	fid := (f := SourceFiles at: 1) fileID.
	[f primSize: fid] bench

answers

	'654,000 per second. 1.53 microseconds per run. 0 % GC time.'

using the old seek here, seek end, seek here method, but answers

	'2,410,000 per second. 414 nanoseconds per run. 0 % GC time.'

using the new fstat method.

Author: eliotmiranda

platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c

@@ -130,6 +130,12 @@ static squeakFileOffsetType
 getSize(SQFile *f)
 {
   FILE *file = getFile(f);
+# if !defined(NO_STD_FILE_SUPPORT)
+  struct stat buf;
+
+  if (!fstat(fileno(file),&buf))
+	return buf.st_size;
+#endif
   squeakFileOffsetType currentPosition = ftello(file);
   fseeko(file, 0, SEEK_END);
   squeakFileOffsetType size = ftello(file);