[sanitizers] Add a blocking boolean to GetRandom prototype

Summary:
On platforms with `getrandom`, the system call defaults to blocking. This
becomes an issue in the very early stage of the boot for Scudo, when the RNG
source is not set-up yet: the syscall will block and we'll stall.

Introduce a parameter to specify that the function should not block, defaulting
to blocking as the underlying syscall does.

Update Scudo to use the non-blocking version.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D36399

llvm-svn: 310839