syscall: optimize UTF16{,Ptr}FromString
syscall: optimize UTF16{,Ptr}FromString

Use bytealg.IndexByteString in UTF16FromString instead of an open-coded
loop.

Change-Id: I366448382f2d0adeca6b254131e0087a1f489258
Reviewed-on: https://go-review.googlesource.com/c/go/+/393614
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go