i#3544 RV64: Enable sample.bbbuf test

[ksco]

This patch enables sample.bbbuf test with the following fixes:

• x0 on RISC-V is like XZR on AArch64, so do not consider it as a general register
• tp should be avoided in drreg_reserve_reg_internal()
• RISC-V does not have control flags, so a dummy hello test is used for control_flags instead

Also removed linux.sigaction_nosignals and pthreads.pthreads from RUNS_ON_QEMU since they are not working on real hardware.

#3544

[ksco]

@abhinav92003 Can you merge this? Thanks.

[abhinav92003]

x86-32 failures are #6417.