Require getrandom(2).

It's almost 2018, Linux 3.17 -- the first kernel with getrandom(2) --
was released in October 2014, and being able to assume getrandom lets
us simplify and improve some security-related code in libc, and revert a
hack to getentropy that's been necessary in the meantime. Only the fugu
kernel doesn't have getrandom(2) at this point, and that's EOL.

Bug: http://b/67014255
Test: ran tests
Change-Id: I1736f4dd33d50fe99e7a524851180989f4c85a33