ffigen: Don't generate setters for constant globals

[simolus3]

ffigen always generates a getter/setter pair for global variables declared in C. But I think for variables declared const in C, writing to their address should be impossible in Dart, just like it is prevented in C.

[dcharkes]

lgtm from my side! Thanks @simolus3!

[dcharkes]

PTAL as well @mannprerak2.

[coveralls]

coverage: 92.677% (+0.008%) from 92.669%
when pulling 74a5df7 on simolus3:constant-globals
into 9f29edc on dart-lang:main.

[mannprerak2]

Looks good @dcharkes 👍