I would say, the best place to ask would be the Keil forum.
But AFAIK, the Keil startup file (as with most toolchains) comes already as assembler code.
For the "main" file, you could start out from a basic C project, and review the generated assembler code.
Not sure if the Keil IDE has a project wizard for assember-based projects, but you can always add assember files to a C project.
Two notes:
The Arm/Keil assembly file syntax used to differ slightly from the GCC toolchain syntax. I think this is still the case.
And second, you might need to explicitely enable the generation of assembly ("intermediate") files in the project settings, many toolchains don't do that by default any more.
