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.

Author: jedbrown

core/src/lib.rs

@@ -192,6 +192,7 @@
 #![feature(hexagon_target_feature)]
 #![feature(loongarch_target_feature)]
 #![feature(mips_target_feature)]
+#![feature(nvptx_target_feature)]
 #![feature(powerpc_target_feature)]
 #![feature(riscv_target_feature)]
 #![feature(rtm_target_feature)]