LLVM mico32 -> lm32

Hi,

do you have any concerns about renaming the LLVM target name from 
"Mico32" to "LM32"?

Binutils use "lm32", and this discrepancy is particularly problematic 
when clang attempts to find programs like "mico32-elf-as" (instead of 
the correct "lm32-elf-as").

Right now we work around this by using the -ccc-gcc-name option (which 
passes the assembler code to GCC, not AS, and therefore requires a full 
GCC install), but I'd like to clean things up.

Regards,
Sébastien