snapshot_util.cc using incorrect guard

[biggs0125]

https://github.com/dart-lang/sdk/blob/main/runtime/bin/snapshot_utils.cc#L221 is using "TARGET" guards rather than "HOST" guards which lead to issues with x-compile builds like: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8697566385942223217/+/u/build_dart/stdout

[biggs0125]

cc @alexmarkov

[alexmarkov]

My understanding is that snapshot_utils.cc contains utility classes and function which implement (1) loading snapshots; (2) writing snapshots. (1) is not supposed to be used in cross-compilation environment (when host != target), while (2) is used by gen_snapshot built as a cross-compiler.

For (1) checking both target and host is fine as they are assumed to be the same, so it is not necessary a bug to check the one or the other. (2) should check target. As long as we're keeping them together my preference to have a consistency between both. The problem is that we're still compiling (1) while building a cross-compiler, so it should compile when host != target. Maybe we should just split this file into 2 separate sources, exclude (1) from cross-compiler and verify that (1) is only compiled when target == host.