Fold CRYPTO_hwrand and CRYPTO_have_hwrand together.

Since the caller must check for CRYPTO_hwrand failures anyway, there's not much
point in doing the CRYPTO_have_hwrand check externally.

(As a bonus, CRYPTO_hwrand no longer compiles to abort() on ARM, so linker
deduplicating won't confuse Chrome's crash reporter...)

Change-Id: I2191d835fbda5b70812f14cd9a873a5e35c30c6d
Reviewed-on: https://boringssl-review.googlesource.com/5630
Reviewed-by: Adam Langley <agl@google.com>