[sanitizer] Add a function to gather random bytes

Summary:
AFAICT compiler-rt doesn't have a function that would return 'good' random
bytes to seed a PRNG. Currently, the `SizeClassAllocator64` uses addresses
returned by `mmap` to seed its PRNG, which is not ideal, and  
`SizeClassAllocator32` doesn't benefit from the entropy offered by its 64-bit
counterpart address space, so right now it has nothing. This function aims at
solving this, allowing to implement good 32-bit chunk randomization. Scudo also
has a function that does this for Cookie purposes, which would go away in a
later CL once this lands.

This function will try the `getrandom` syscall if available, and fallback to
`/dev/urandom` if not.

Unfortunately, I do not have a way to implement and test a Mac and Windows
version, so those are unimplemented as of now. Note that `kRandomShuffleChunks`
is only used on Linux for now.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: zturner, rnk, llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34412

llvm-svn: 305922