add Silicon Laboratories ignore file

added a gitignore file for Silicon Labs IDE, which is used for C8051 (C or ASM).
* latest version: [Silicon Labs](http://www.silabs.com/products/mcu/Pages/simplicity-studio.aspx)

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Silicon Labs IDE is widely used in our school. I used this ignore file in projects, so I contribute to share it with others.
[a example repo using Silicon Labs IDE (mine)](https://github.com/DefineFC/eSys2011)

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* *.HEX & *.OBJ are known ignorable files
* *.#* is generated when compiling
* tmp.out is a temp file for Silicon Labs
* *.LST is list filetype, it is also ignored in `Tasm.gitignore `
* *.M51 is generated when compiling, it stores the stack status