riscv: Remove ECALL tests from insttest

riscv: Remove ECALL tests from insttest

The system calls tested in rv64i.cpp in RISC-V's insttest suite have
different behavior depending on the operating system and file system they
are run on. This patch ignores the output of those tests and only
ensures that the instructions in RV64I complete successfully.

[Change deletion of ECALL test to block comment.]
[Restore ECALL test but remove test output to test only for completion
without error.]
[Update patch description and again try to push EMPTY files for rv64i
tests.]