[llvm-exegesis] Set stack pointer register after starting perf counter (#72489)

Before this patch, in subprocess mode, llvm-exegesis setup the stack
pointer register with the rest of the registers when it was requested by
the user. This would cause a segfault when the instructions to start the
perf counter ran as they use the stack to preserve the three registers
needed to make the syscall. This patch moves the setup of the stack
register to after the configuration of the perf counter to fix this
issue so that we have a valid stack pointer for all the preceeding
operations.

Regression test added.

This fixes #72193.