sigchain: print ucontext in fatal error log

Because it's hard to debug when ucontext is optimized,
to print ucontext in fatal error log.

gdb log:
 #1  0xa98b52be in art::SignalChain::Handler (signo=11, siginfo=0xae4cec88, ucontext_raw=<optimized out>)
    at art/sigchainlib/sigchain.cc:408

Test: trigger fatal error by sending signal 11 to a process
Change-Id: I204c308e5d1d0b8d12ab15045a7426e64ccd8e2e
Bug: 197044435
1 file changed