Do not assume "strip all symbols" when strip(1) -x option is specified.

Ticket:	      #430
Reported by:  rsherloc

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2968 95820547-d848-0410-985e-9ae8fe0fa350
When stripping local symbols (option -x for strip(1)), do not strip
local debugging symbols.

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2969 95820547-d848-0410-985e-9ae8fe0fa350