i#3544 RV64: Enable more tests on CI

[ksco]

Enabling passed tests on RISC-V. Some extra work is necessary:

• Implements insert_parameter_preparation() and mangle_syscall_arch() by shared them with AARCHXX.
• Add currently unused SCRATCH_REG5 -- this allows us to share the code with AARCHXX more easily.
• Implements mangle_insert_clone_code()

Issue: #3544

[ksco]

x86-32-ubuntu22 tool.drcacheoff.simple #6416
x86-64-ubuntu22 api.rseq #6185

[ksco]

@derekbruening vs2019-64 is failing because: There is not enough space on the disk.

[ksco]

Oh looks like vs2019-64 is running on a standard GA runner (Windows Server 2019), not a self-hosted one, so this could be an issue on Github side.

[ksco]

x86-32-ubuntu22 tool.drcacheoff.simple failed 5 times in a row: #6416