handle EWOULDBLOCK error in _dispatch_operation_perform
handle EWOULDBLOCK error in _dispatch_operation_perform

POSIX.1-2001 allows either EAGAIN or EWOULDBLOCK to be returned when a
write to a nonblocking socket would block. For example IBM z/OS returns
EWOULDBLOCK for this case.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch