Add an option for using '-fwhopr' instead of '-flto'

[Artoria2e5]

This splits up the LTO work (potentially across cores), and makes LTO'ing big programs less painful.

https://gcc.gnu.org/onlinedocs/gccint/LTO-Overview.html

[MingcongBai]

I will test this out later today to see if there is some likelihood for build failures. If -fwhole-program is able to be used in a single thread environment, this should be enabled by default.