If exitsyscall tries to grow the stack it will panic, but throw calls
print, which can grow the stack. Move the two bare throws in
exitsyscall to the system stack.

Updates #21431.

Change-Id: I5b29da5d34ade908af648a12075ed327a864476c
Reviewed-on: https://go-review.googlesource.com/79517
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>