syscall: add //go:build lines to files generated with with mksyscall_libc.pl
syscall: add //go:build lines to files generated with with mksyscall_libc.pl

Change-Id: I2e02d02d9208fc2dbf01c0cea4a67c288967cd07
Reviewed-on: https://go-review.googlesource.com/c/go/+/315276
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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