[arm] use target_mgreg_t instead of plain pointer type in sdb breakpoint (mono/mono...

[arm] use target_mgreg_t instead of plain pointer type in sdb breakpoint (mono/mono#15205)

[arm] use target_mgreg_t instead of plain pointer type in sdb breakpoint

Fixes Debug mode for iOS 32bit.

Contributes to https://github.com/mono/mono/issues/15058 & https://github.com/mono/mono/issues/9621

Diff before (left old cross32, right new cross32 running on amd64 host _before_ this fix):
<img width="1257" alt="Screenshot 2019-06-17 at 14 09 51" src="https://user-images.githubusercontent.com/75403/59603413-a9bd4f00-9109-11e9-8f18-aa7b4ca5817f.png">

/cc @mandel-macaque

Commit migrated from https://github.com/mono/mono/commit/d5a35798c620ebd47d7a317014c4f32974d0a748