devel/riscv64-gcc: Switch upstream to official GCC and bump version to 8.3.0

[mhorne]

Summary

RISC-V support landed in GCC officially in version 7.1, so we should no
longer need to maintain our own fork. While here, bump to the latest GCC
8 release, and make some very minor cleanups.

Test Plan

Port builds in poudriere and can compile RISC-V world + kernel.

[lwhsu]

Thanks for the patch. I've tested the result with qemu and changed to use .xz distfile.