Restore errno in signal handlers.

There was a problem where errno could be set in a signal handler
and cause bugs if other system calls were interrupted.

There isn't strong evidence this is causing any issues, but add this
proactively.

Bug: 31448909

Test: Ran the backtrace unit tests, backtraces few random processes,
Test: forced the ANR path for some zygote based processes.
Test: Ran the art ThreadStress test a few times.
Change-Id: I5abc9e330a5e86cea7015e2362f66044c6bc37d0