[llvm-exegesis] Print generated assembly snippet (#142540)

Debug generated disassembly by passing argument
`debug-only="print-gen-assembly"` or `debug-only=preview-gen-assembly`
of exegesis call.
`--debug-only="print-gen-assembly"` debugs the whole generated assembly
snippet .
`--debug-only=preview-gen-assembly` debugs the initial 10 instructions
and ending 3 lines.
Thus, We can in glance see the initial setup code like registers setup
and instruction followed by truncated middle and finally print out the
last 3 instructions.

This helps us look into assembly that exegesis is execution in hardware,
Thus, it is simply functionally alias to separate objdump command on the
dumped object file.