gdb: tidy RISC-V part in gdb/Makefile.in and gdb/configure.tgt

I noticed that the length of line in gdb/configure.tgt was more than
80 lines, so I straightened it out. Also, in gdb/Makefile.in newly added
file riscv-linux-canonicalize-syscall-gen.c was missed in ALLDEPFILES,
so I added it there. And the last change: one more file
riscv-linux-canonicalize-syscall-gen.o was not in alphabetical order.