Fix GHCi on Arm (#10375).
Arm has two instruction sets, Arm and Thumb, and an execution mode for each. Executing Arm code in Thumb mode or vice-versa will likely result in an Illegal instruction exception. Furthermore, Haskell code compiled via LLVM was generating Arm instructions while C code compiled via GCC was generating Thumb code by default. When these two object code types were being linked by the system linker, all was fine, because the system linker knows how to jump and call from one instruction set to the other. The first problem was with GHCi's object code loader which did not know about Thumb vs Arm. When loading an object file `StgCRun` would jump into the loaded object which could change the mode causing a crash after it returned. This was fixed by forcing all C code to generate Arm instructions by passing `-marm` to GCC. The second problem was the `mkJumpToAddr` function which was generating Thumb instructions. Changing that to generate Arm instructions instead results in a working GHCi on Arm. Test Plan: validate on x86_64 and arm Reviewers: bgamari, austin, hvr Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1323 GHC Trac Issues: #10375
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