Make it possible to disable method body rewriting

[MichalStrehovsky]

Linker should respect the NoOptimization MethodImpl flag and it should prevent it from messing with instructions in method bodies. Would make it easier to work around issues like #2181 or dotnet/runtime#57010 that now require more creative workarounds.

I'm putting this in 6.0 because the method body rewriting is a relatively new feature, we're still finding bugs in it, and we'll likely find more when 6.0 ships.

[vargaz]

Would be nice to have a linker/msbuild option to turn this off as well so users can work around it if this happens in 3rd party/bcl code.

[MichalStrehovsky]

Setting the trimmode to copy on the problematic assembly might be good enough workaround for 6.0.