RTS : Fix StgRun for aarch64-linux (#9935).

Summary:
The GCC assembler doesn't seem to recognise the 'fp' and 'lr' register
names which are aliases for 'x29' and 'x30' respectively.

Depends on D598.

Test Plan: validate

Reviewers: lukexi, bgamari, austin

Reviewed By: austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D599

GHC Trac Issues: #9935

(cherry picked from commit b9063703)