The below specifically address portablity in microcontroller systems, for microprocessor systems it does not apply.

This will make your code ‘Only for 8051’ & you will loose advantage of using C that is portability
I had the pleasure once to do a project where 9 different models were made with increasing richness of features. To accomodate the pricing structure the low end (highest volume) was made with a '51 and the high end with a Philips XA (big developent cost, but so what, we made 680.000 of the thingies). This gave me some insight in "portability" and, ladies and gentlemen, mainly it is a myth. Yes, we ended up being able to use about 70% of the code on both platforms through some very hard structuring and making two BIOS with "device drivers" that allowed most application routines "not to know" which processor they were running on.

HOWEVER, "portability" as an automatic result of C IS a myth.
We had to spend hundereds of hours to make the code "portable" (i.e. able to be used on both platforms), but if we had not designed based on "darn it this code MUST run on both" there would have been no "portability".

So, it requires a significant effort to make microcontroller C portable and I doubt such an effort is made in 99% of the cases.

Erik