[opt] Enable -mcpu=help without an input file (#187876)
This patch enables `-mcpu=help` or `-mattr=help` invocation without an
input file, like `llc`. For example, the following command `opt
-mtriple=aarch64 -mattr=help` would previously hang but now it succeeds.
The implementation is similar to llc, creating a target machine for help
printing side effects and existing early. Note: llc has a bug in the
default triple handling that we already fix here upfront. Ill submit a
separate fix for llc.
