While understanding why syscall.Read is 2x slower on darwin/amd64, I found
out that, contrary to popular belief, the slowdown is not due to the migration
to use libSystem.dylib instead of direct SYSCALLs, i.e., CL 141639 (and #17490),
but due to a subtle change introduced in CL 141639.

Previously, syscall.Read used syscall.Syscall(SYS_READ), whose preamble called
runtime.entersyscall, but after CL 141639, syscall.Read changes to call
runtime.syscall_syscall instead, which in turn calls runtime.entersyscallblock
instead of runtime.entersyscall. And the entire 2x slow down can be attributed
to this change.

I think this is unnecessary as even though syscalls like Read might block, it
does not always block, so there is no need to handoff P proactively for each
Read. Additionally, we have been fine with not handing off P for each Read
prior to Go 1.12, so we probably don't need to change it. This changes restores
the pre-Go 1.12 behavior, where syscall preamble uses runtime.entersyscall,
and we rely on sysmon to take P back from g blocked in syscalls.

Change-Id: If76e97b5a7040cf1c10380a567c4f5baec3121ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/197938
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>