syscall: simplify and optimize environment block creation on Windows

syscall: simplify and optimize environment block creation on Windows

createEnvBlock currently allocates multiple times: at least one to
convert the slice of strings into a NULL separated slice of bytes, and
then again to encode it as UTF-16. The logic to do so is also quite
complex.

This CL simplifies the logic by allocating only once by encoding the
slice of strings into UTF-16 directly using utf16.AppendRune.

goos: windows
goarch: amd64
pkg: syscall
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
                  │   old.txt    │               new.txt               │
                  │    sec/op    │   sec/op     vs base                │
CreateEnvBlock-12   37.92µ ± 24%   21.36µ ± 8%  -43.66% (p=0.000 n=10)

                  │    old.txt    │               new.txt                │
                  │     B/op      │     B/op      vs base                │
CreateEnvBlock-12   109.12Ki ± 0%   26.62Ki ± 0%  -75.60% (p=0.000 n=10)

                  │  old.txt   │              new.txt               │
                  │ allocs/op  │ allocs/op   vs base                │
CreateEnvBlock-12   4.000 ± 0%   1.000 ± 0%  -75.00% (p=0.000 n=10)

Change-Id: If35f62c3926b486d5253a9ae23a33b979b2f02c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/531355
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>