riscv: rename to riscv64

riscv: rename to riscv64

The only currently supported RISC-V architecture is the 64-bit RISC-V.
As the generic name we use causes confusion [1], rename it to riscv64.

[1] https://lists.strace.io/pipermail/strace-devel/2019-August/009057.html

* Makefile.am (EXTRA_DIST): Rename linux/riscv to linux/riscv64.
* configure.ac: Rename riscv to riscv64, RISCV to RISCV64.
* linux/riscv: Rename to linux/riscv64.
* riscv.c: Rename RISCV to RISCV64.
* tests/options-syntax.test: Rename riscv to riscv64.
* tests/qualify_personality.sh: Likewise.
* tests/strace-V.test: Likewise.