scons: Add a priority field to the SourceLib construct.

This helps specify ordering for libraries that need it. Libraries with
a higher priority will be sorted earlier in the list, which can be
necessary when working with static libraries/archives.

The default value for "priority" is zero. It's only really necessary
to ensure relative ordering of particular pairings of libraries, so
it should be ok to use an absolute integer value for this. If you
need to order relative to a library, there is a good chance you're
adding it, or the place it's added is well known and you can easily
find its priority value. It's also unlikely that there would be a
complex series of interactions between libraries that would make a
more complicated system warranted.

Change-Id: Ie94a35e6563c07f8d462a4a52d0173ea3cf4f8de
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58350
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jui-min Lee <fcrh@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>

src/SConscript

@@ -76,7 +76,9 @@ class Source(SourceFile):
     pass
 
 class SourceLib(SourceItem):
-    pass
+    def __init__(self, *args, **kwargs):
+        self.priority = kwargs.pop('priority', 0)
+        super().__init__(*args, **kwargs)
 
 build_tools = Dir('#build_tools')
 
@@ -388,6 +390,8 @@ class Executable(TopLevelBase):
                 env['BUILDDIR'], self.path(env).dir.abspath)
 
         libs = self.libs(env)
+        # Higher priority libraries should be earlier in the list.
+        libs.sort(key=lambda l: l.priority, reverse=True)
         if libs:
             env.Append(LIBS=list(lib.source for lib in libs))