[Support] Make the default chunk size of raw_fd_ostream to 1 GiB.

Previously, we call write(2) for each 32767 byte chunk. That is not
efficient because Linux can handle much larger write requests.
This patch changes the chunk size on Linux to 1 GiB.

This patch also changes the default chunks size to SSIZE_MAX. I think
that doesn't in practice change this function's behavior on any operating
system because SSIZE_MAX on 64-bit machine is unrealistically large,
and writing 2 GiB (SSIZE_MAX on 32-bit) on a 32-bit machine by a single
call of write(2) is also unrealistic, as the userspace is usually
limited to 2 GiB. That said, it is in general a good thing to do because
a write larger than SSIZE_MAX is implementation-defined in POSIX.

Differential Revision: https://reviews.llvm.org/D39444

llvm-svn: 317015