cmd/compile: implement float min/max in hardware for riscv64

cmd/compile: implement float min/max in hardware for riscv64

CL 514596 adds float min/max for amd64, this CL adds it for riscv64.

The behavior of the RISC-V FMIN/FMAX instructions almost match Go's
requirements.

However according to RISCV spec 8.3 "NaN Generation and Propagation"
>> if at least one input is a signaling NaN, or if both inputs are quiet
>> NaNs, the result is the canonical NaN. If one operand is a quiet NaN
>> and the other is not a NaN, the result is the non-NaN operand.

Go using quiet NaN as NaN and according to Go spec
>> if any argument is a NaN, the result is a NaN

This requires the float min/max implementation to check whether one
of operand is qNaN before float mix/max actually execute.

This CL also fix a typo in minmax test.

Benchmark on Visionfive2
goos: linux
goarch: riscv64
pkg: runtime
         │ float_minmax.old.bench │       float_minmax.new.bench        │
         │         sec/op         │   sec/op     vs base                │
MinFloat             158.20n ± 0%   28.13n ± 0%  -82.22% (p=0.000 n=10)
MaxFloat             158.10n ± 0%   28.12n ± 0%  -82.21% (p=0.000 n=10)
geomean               158.1n        28.12n       -82.22%

Update #59488

Change-Id: Iab48be6d32b8882044fb8c821438ca8840e5493d
Reviewed-on: https://go-review.googlesource.com/c/go/+/514775
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Run-TryBot: M Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>