Allow property += value in ld.config.txt

ld.config.txt currently does not support split line. As the file gets
larger, this limitation makes the file very unreadable. Now, long lines
can be avoided by breaking one line into multiple lines using +=
operator.

ex)

namespace.default.search.paths = /system/${LIB}
namespace.default.search.paths += /system/${LIB}

Delimitor (':' for *.paths and *.shared_libs, and ',' for *.links) is
automatically added.

Bug: 69888716
Test: linker-unit-tests passes

Change-Id: I4b94fd4e7f8a76d59db8d1096c86aa2118e46625