feat: remove redundant codes under virt64, c906

Changes:

- create new folder name `common64` and save all common codes of rv64
  inside

Signed-off-by: Shell <smokewood@qq.com>

libcpu/risc-v/common64/SConscript

@@ -0,0 +1,12 @@
+from building import *
+
+cwd     = GetCurrentDir()
+src     = Glob('*.c') + Glob('*.cpp') + Glob('*_gcc.S')
+CPPPATH = [cwd]
+
+if not GetDepend('ARCH_USING_ASID'):
+    SrcRemove(src, ['asid.c'])
+
+group = DefineGroup('libcpu', src, depend = [''], CPPPATH = CPPPATH)
+
+Return('group')