Rename `context` to `ctx` in kernel code

Summary:
Simple change to standardise the `RuntimeContext` to be stored in the variable name `ctx` in kernel code (previously `context`).

This diff was generated with the following commands:

```
cd ~/fbsource/xplat/executorch/kernels/portable/cpu
perl -i~ -0777 -pe "s/RuntimeContext&\s+context/RuntimeContext& ctx/g" *.cpp
perl -i~ -0777 -pe "s/\(void\)\s*context/(void)ctx/g" *.cpp
```

Reviewed By: manuelcandales

Differential Revision: D48204823

fbshipit-source-id: 1c126c04295e20774fe81f09cc1dc9945c2bdfff