Generic-ELF: Add platform module to configure the .elf file extension
The `.elf` extension is a common convention used for embedded binaries.
Both Arm and RISC-V use the ELF file format for executables.
Configuring the `.elf` file extension is typically done incorrectly.
Most embedded developers set this in toolchain files, which is not
the correct place.
This is typically accomplished through a hack by setting the individual
language file extensions for C, CXX, and ASM.
Multiple CMake issues in the past have been opened related to this.
* #16538
* #20163
* #17880
A platform module makes it simpler and less error prone for developers
targeting these ubiquitous bare-metal platforms.
This PR attempts to solve this globally with a generic platform.
This could also be solved by using more specific platform modules, such
as one for each of bare-metal Arm and RISC-V.
Jordan Williams