Stop exporting symbols from app_process

Any symbols exported from the executable override everything else in
every linker namespace. Previously, app_process exported the
signal/sigchain API, but that interposition is now handled by
libsigchain.so.

Bug: http://b/190100879
Test: `nm -D --defined-only app_process{32,64}` shows no symbols
Change-Id: I1ab1fc700c34e91535c3e679a471debbb4eb71e3