Add riscv64 support

[Avimitin]

Description

In riscv64, the syscall.Uname function will return a uint8 slice. This is tested in the Arch Linux RISC-V 64 QEMU environment.

func main() {
      var buf syscall.Utsname
	  syscall.Uname(&buf)
      fmt.Printf("Buffer Type: %T\n", buf.Release)
}

$ go build . && ./test
Buffer Type: [65]uint8
$ uname -m
riscv64

Motivation and Context

I am now trying to make minio run on Arch Linux RISC-V, and this package fail to build because of the utsnameStr is not compile. This patch should help improve this issue.

How to test this PR?

You can follow this instruction to setup a QEMU environment https://github.com/felixonmars/archriscv-packages/wiki/Setup-Arch-Linux-RISC-V-Development-Environment.

Then copy the source code into the qemu machine and run:

pacman -Syu go
cd minio
go build .

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
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