scons: Streamline the LTO option setting code and add options for clang.
scons: Streamline the LTO option setting code and add options for clang.

Clang also supports an -flto option, although not with a thread count.

Change-Id: Ifc783ac3ab2e0ea93fad0402b288605a20725aa0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40962
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5