Support for `linux/riscv64`

Hi,

I think it will be great if modernc.org/libc work in Linux RISC-V (64-bit) environment.

I've done some basic work:

• piggynl/cznic-cc@master...linux-riscv64
• piggynl/cznic-libc@master...linux-riscv64

In make linux_riscv64, go generate works fine now, but go build produces this:

package modernc.org/libc
        imports modernc.org/libc/uuid/uuid: build constraints exclude all Go files in /home/piggy/libc/uuid/uuid

There are Code generated by 'ccgo uuid/gen.c ...' in the beginning of the go files in the uuid directory, but gen.c seems not exist.

Where can I find the uuid/gen.c? And what else can I do? Thank you for your work!

Also, if you are interested, I can help you setup a QEMU environment and/or provide you ssh access to a SiFive Unmatched RISC-V development board (the one that ci.rvperf.org uses).