sys/ps: don't use float for runtime calculations

Not sure if I'd prefer `%` or `ppm`..

%:
```
> ps
	pid | name                 | state    Q | pri | stack ( used) location  | runtime | switches
	  0 | idle                 | pending  Q |  31 |  8192 ( 1088) 0x8057254 |     99% |         1
	  1 | main                 | running  Q |  15 | 16384 ( 2840) 0x8059254 |      0% |         8
	  2 | uart0                | bl rx    _ |  14 |  8192 (  924) 0x805de0c |      0% |        10
	  3 | radio                | bl rx    _ |  13 |  8192 (  896) 0x806abe0 |      0% |         1
	  4 | Transceiver          | bl rx    _ |  12 | 16384 (  900) 0x806de64 |      0% |         1
	    | SUM                  |            |     | 57344
```

ppm:
```
> ps
	pid | name                 | state    Q | pri | stack ( used) location  | runtime | switches
	  0 | idle                 | pending  Q |  31 |  8192 ( 1088) 0x8057254 |  999ppm |         1
	  1 | main                 | running  Q |  15 | 16384 ( 2840) 0x8059254 |    0ppm |         8
	  2 | uart0                | bl rx    _ |  14 |  8192 (  924) 0x805de0c |    0ppm |        10
	  3 | radio                | bl rx    _ |  13 |  8192 (  896) 0x806abe0 |    0ppm |         1
	  4 | Transceiver          | bl rx    _ |  12 | 16384 (  900) 0x806de64 |    0ppm |         1
	    | SUM                  |            |     | 57344
```
Damn, you're good ;)