Add LD_PRELOAD for libsigchain.so to the environment

To remove the need to modify the bionic dynamic linker, add the
signal chaining library as a preload in the environment.  This
will be picked up by the dynamic linker and will override
sigaction and sigprocmask to allow for signal chaining.

Change-Id: I6e2d0628b009bd01e0ed9aed0b311871b9c8363a