Support for semihosting

It would be nice to allow support for semihosting for targets that have no serial output possibilities. In order to achieve that, one needs to add **--specs=rdimon.specs** to the linker options. But then since the low-level functions defined in rdimon.specs are already defined in retarget.cpp, the program will not compile or work. These functions are:
- _lseek
- _isatty
- _open
- _close

I can see that in device.h file of some mbed-hal-\* modules, **DEVICE_SEMIHOST** exists but it doesn't seem to have been used at all. One suggestion that I've tried and worked is to #if !DEVICE_SEMIHOST the definitions of those functions out. The two functions below are also necessary to exclude for semihosting to work correctly:
- _read
- _write

Start with retarget.cpp and the device.h files in the mbed-hal-* modules, then inspect how DEVICE_SEMIHOST is defined or consumed. Verify the --specs=rdimon.specs linker path and the listed low-level functions, including _read and _write. Done means semihosting targets compile and work without conflicting retarget definitions, while other targets retain their normal serial behavior.

Written by the indexing model from the issue text.