Move to preferring getentropy() for system provided entropy

This changes the order of things so that by default your
system is expected to provide us with a getentropy() in
<unistd.h> for integrators that are not explicitly
supported.

We preserve the getrandom/urandom dance for Linux and Android
for now.

Linux has had getentropy() in libc's since 2017
macOS, and all the BSD's have had it for any versions we
care about.

iOS hides it from us - so we use CommonCrypto CCRandomGenerateBytes

Update-Note: Non-macOS Apple platforms now use CCRandomGenerateBytes
instead of /dev/urandom. Linux behavior remains unchanged. Platforms
which were not explicitly supported with a different codepath will also
switch from /dev/urandom to getentropy. If your platform specifically
requires /dev/urandom, let us know.know

Bug: 287
Change-Id: I0c2b8c594c473e4395379f50b0c4e6713c0a4c02
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61325
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>

crypto/CMakeLists.txt

diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index 7bc73b1..7c944c1 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -200,6 +200,8 @@
   rand_extra/deterministic.c
   rand_extra/forkunsafe.c
   rand_extra/fuchsia.c
+  rand_extra/getentropy.c
+  rand_extra/ios.c
   rand_extra/passive.c
   rand_extra/rand_extra.c
   rand_extra/windows.c