Add support for *.S and *.s files

[triztian]

AVR GCC assembler files have a *.S and *.s extension, see here for more information:

• https://www.microchip.com/webdoc/AVRLibcReferenceManual/assembler_1ass_tools.html

Excerpt from the previous link:

Note that the invokation of the C preprocessor will be automatic when the filename provided for the assembler file ends in .S (the capital letter "s"). This would even apply to operating systems that use case-insensitive filesystems since the actual decision is made based on the case of the filename suffix given on the command-line, not based on the actual filename from the file system.