Avoid buffered reads of /dev/urandom

I am seeing disappointing clang performance on a large PowerPC64
Linux box. GetRandomNumberSeed() does a buffered read from
/dev/urandom to seed its PRNG. As a result we read an entire page
even though we only need 4 bytes.

With every clang task reading a page worth of /dev/urandom we
end up spending a large amount of time stuck on kernel spinlock.

Patch by Anton Blanchard!

llvm-svn: 255386