add nvptx_target_feature

Add target features for sm_* and ptx*, both of which form a partial
order, but cannot be combined to a single partial order. These mirror
the LLVM target features, but we do not provide LLVM target
processors (which imply both an sm_* and ptx* feature).

Add some documentation for the nvptx target.

compiler/rustc_span/src/symbol.rs

@@ -1509,6 +1509,7 @@ symbols! {
         not,
         notable_trait,
         note,
+        nvptx_target_feature,
         object_safe_for_dispatch,
         of,
         off,