RISCV64_GENERIC: Use generic kernel for DSDOT for better precision

The implementation in `riscv64/dot.c` fails the `test_dsdot` test, and
the generic kernel seems to have better precision. Tested on SiFive
FU740 (HiFive Unmatched) and QEMU.

Also see #1469.

kernel/riscv64/KERNEL.RISCV64_GENERIC

@@ -54,6 +54,7 @@ SDOTKERNEL   = ../riscv64/dot.c
 DDOTKERNEL   = ../riscv64/dot.c
 CDOTKERNEL   = ../riscv64/zdot.c
 ZDOTKERNEL   = ../riscv64/zdot.c
+DSDOTKERNEL  = ../generic/dot.c
 
 SNRM2KERNEL  = ../riscv64/nrm2.c
 DNRM2KERNEL  = ../riscv64/nrm2.c