Fix "integer constant is too large for 'long' type" errors.

(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>