Add new #[target_feature = "..."] attribute.
This commit adds a new attribute that instructs the compiler to emit target specific code for a single function. For example, the following function is permitted to use instructions that are part of SSE 4.2: #[target_feature = "+sse4.2"] fn foo() { ... } In particular, use of this attribute does not require setting the -C target-feature or -C target-cpu options on rustc. This attribute does not have any protections built into it. For example, nothing stops one from calling the above `foo` function on hosts without SSE 4.2 support. Doing so may result in a SIGILL. This commit also expands the target feature whitelist to include lzcnt, popcnt and sse4a. Namely, lzcnt and popcnt have their own CPUID bits, but were introduced with SSE4.
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