5/llvm: Add LLVM as a package for RTEMS.

- Add '%source download <source>' to only download the source
  and do not unpack and prep. This can used when a package
  internally needs another source package.

- Install the staging root only if it is present. A package
  may internally build another package that is not staged as it
  is not suitable for installing.

Updates #3250
Updatew #3797

source-builder/sb/options.py

@@ -293,6 +293,8 @@ class command_line:
                               '--without-log',
                               '--without-error-report',
                               '--without-release-url']
+            if a == '--dry-run':
+                self.args += ['--without-error-report']
             arg += 1
 
     def post_process(self, logfile = True):