gn build: Add NVPTX target

The NVPTX target is a bit unusual in that it's the only target without a
disassembler, and one of three targets without an asm parser (and the
first one of those three in the gn build). NVPTX doesn't have those
because it's not a binary format.

The CMake build checks for the existence of
{AsmParser,Disassembler}/CMakeLists.txt when setting
LLVM_ENUM_ASM_PARSERS / LLVM_ENUM_DISASSEBLERS
(http://llvm-cs.pcc.me.uk/CMakeLists.txt#744). The GN build doesn't want
to hit the disk for things like this, so instead I'm adding explicit
`targets_with_asm_parsers` and `targets_with_disassemblers` lists. Since
both are needed rarely, they are defined in their own gni files.

Differential Revision: https://reviews.llvm.org/D63210

llvm-svn: 363437