nvptx: Add -misa=sm_75 and -misa=sm_80

Add new target macros TARGET_SM75 and TARGET_SM80.  Add support for
__builtin_tanhf, HFmode exp2/tanh and also for HFmode min/max, controlled by
TARGET_SM75 and TARGET_SM80 respectively.

The following has been tested on nvptx-none, hosted on x86_64-pc-linux-gnu
with a "make" and "make -k check" with no new failures.

gcc/ChangeLog:

	* config/nvptx/nvptx-opts.h (ptx_isa): PTX_ISA_SM75 and PTX_ISA_SM80
	ISA levels.
	* config/nvptx/nvptx.opt: Add sm_75 and sm_80 to -misa.
	* config/nvptx/nvptx.h (TARGET_SM75, TARGET_SM80):
	New helper macros to conditionalize functionality on target ISA.
	* config/nvptx/nvptx-c.c (nvptx_cpu_cpp_builtins): Add __PTX_SM__
	support for the new ISA levels.
	* config/nvptx/nvptx.c (nvptx_file_start): Add support for TARGET_SM75
	and TARGET_SM80.
	* config/nvptx/nvptx.md (define_c_enum "unspec"): New UNSPEC_TANH.
	(define_mode_iterator HSFM): New iterator for HFmode and SFmode.
	(exp2hf2): New define_insn controlled by TARGET_SM75.
	(tanh<mode>2): New define_insn controlled by TARGET_SM75.
	(sminhf3, smaxhf3): New define_isnns controlled by TARGET_SM80.

gcc/testsuite/ChangeLog:

	* gcc.target/nvptx/float16-2.c: New test case.
	* gcc.target/nvptx/tanh-1.c: New test case.