Pull constant-time methods out to a separate header, add tests.

(Imported from upstream's 9a9b0c0401 and
27739e9265)

Change-Id: Id570f5c531aca791112929e6258989f43c8a78d7

Chromium does not like static initializers, and the CPU logic uses one to
initialize CPU bits. However, the crypto library lacks an explicit
initialization function, which could complicate (no compile-time errors)
porting existing code which uses crypto/, but not ssl/.

Add an explicit CRYPTO_library_init function, but make it a no-op by default.
It only does anything (and is required) if building with
BORINGSSL_NO_STATIC_INITIALIZER.

Change-Id: I6933bdc3447fb382b1f87c788e5b8142d6f3fe39
Reviewed-on: https://boringssl-review.googlesource.com/1770
Reviewed-by: Adam Langley <agl@google.com>

Thanks to Denis Denisov for running the analysis.

Change-Id: I80810261e013423e746fd8d8afefb3581cffccc0
Reviewed-on: https://boringssl-review.googlesource.com/1701
Reviewed-by: Adam Langley <agl@google.com>

(Based on Piotr Sikora's change:
https://boringssl-review.googlesource.com/#/c/1361)

Change-Id: I7b62b81f4e4ef3064eee1b39334dc2e50d17f163
Reviewed-on: https://boringssl-review.googlesource.com/1641
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)