syscall: on wasm, do not panic if "process" global is not defined

syscall: on wasm, do not panic if "process" global is not defined

When running wasm in the browser, the "process" global is not defined.
This causes functions like os.Getpid() to panic, which is unusual.
For example on Windows os.Getpid() returns -1 and does not panic.

This change adds a dummy polyfill for "process" which returns -1 or an
error. It also extends the polyfill for "fs".

Fixes #34627
Replaces CL 199357

Change-Id: Ifeb12fe7e152c517848933a9ab5f6f749896dcef
Reviewed-on: https://go-review.googlesource.com/c/go/+/199698
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>