Add a method to leave the symbol table in a library.

When LOCAL_STRIP_MODULE := keep_symbols is set, then the normal strip rules
will be modified so that only the .debug_* sections are removed. The original
symbol table is left alone.

This allows the compilation of certain libraries so that libbacktrace library
can provide meaningful names to functions.

Bug: 12958251
Change-Id: I82bdc304a463012e29086325ccb51163464cb4a9