Allow internal use of stdsimd from detect_feature

[tmiasko]

This allows using feature detection macros, without placing a
requirement of enabled stdsimd feature gate from end users.

A follow-up to changes from #1311, which introduced the new macro.

Noticed while preparing a bump of stdarch submodule in the standard library.

[rust-highfive]

r? @Amanieu

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[Amanieu]

What exactly is this enabling? I'm concerned about accidentally stabilizing something we don't want to stabilize here.

[tmiasko]

This allows the macro to internally use stdsimd feature, without requiring
end users to enable it. This is necessary to use already stable macros, without
requiring stdsimd feature gate.

Note that this implementation strategy is in place since #1303, and this pull
request merely extends the existing approach to a new macro. I presumed this
implementation strategy was already "blessed".

This change surfaces the same stability concerns as #1303. It makes stdsimd
feature gate ineffective from within macros. The only reason this seems fine to
me, is that things happen to be organized such that the generated macro itself
requires stdsimd feature gate whenever more fine grained target features
require it.