Add riscv test benchmarks

[mininny]

Currently, we're running riscv tests from this test vectors: https://github.com/riscv-software-src/riscv-tests.
There's also benchmark test vectors for the vm https://github.com/riscv-software-src/riscv-tests/tree/master/benchmarks, which are commented out in our tests.

asterisc/rvgo/test/vm_test.go

Line 161 in e1a5b01

//runTestCategory("benchmarks") TODO benchmarks (fix ELF bench data loading and wrap in Go benchmark?)

In order to run these benchmark tests, we need to fix the elf of asterisc to correctly handle the different Prog.Type in benchmark elf files.

asterisc/rvgo/fast/elf.go

Lines 40 to 50 in e1a5b01

	if prog.Filesz != prog.Memsz {
	if prog.Type == elf.PT_LOAD {
	if prog.Filesz < prog.Memsz {
	r = io.MultiReader(r, bytes.NewReader(make([]byte, prog.Memsz-prog.Filesz)))
	} else {
	return nil, fmt.Errorf("invalid PT_LOAD program segment %d, file size (%d) > mem size (%d)", i, prog.Filesz, prog.Memsz)
	}
	} else {
	return nil, fmt.Errorf("program segment %d has different file size (%d) than mem size (%d): filling for non PT_LOAD segments is not supported", i, prog.Filesz, prog.Memsz)
	}
	}

After fixing the ELF loading, wrap these benchmark tests in go benchmark test suite, so we can actually view the benchmarks.