Sleep manager tracing strips the path from filenames and uses the result as an
identifier to track drivers that unlock/lock sleep tracing. Replace the function
that strips the path from the string, replace this function with a new macro,
__FILENAME__ which performs the same action in a compiler specific manner.

- GCC_ARM, use __builtin_strrchr which is optimized out at compile time.
- ARM, use __MODULE__ which returns the filename without path.
- IAR, specifiy the --no_path_in_file_macros compiler flag.

…filepath.

The use of __FILE__ macro to get a usable identifier from the driver path
causes the path of the file to be stored in the .text region of the binary.
Given that this remains for the entire duration of the program, storing a
pointer to this string as an identifier is more efficient than copying the
contents of the string during lookup/insertion.

- Move macro definition to mbed_toolchain.h
- Remove double underscores from macro which are reserved.
- Fix macro for IAR until compiler flags to disable path are added again.