[llvm] [llvm-ifs] Handle more e_machine values for --target (PR #128559)

[Roland McGrath via llvm-commits]

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@@ -2018,6 +2019,9 @@ uint16_t convertArchNameToEMachine(StringRef Arch);
 /// Convert an ELF's e_machine value into an architecture name.
 StringRef convertEMachineToArchName(uint16_t EMachine);
 
+// Convert a triple's architecture to ELF's e_machine value.
+uint16_t convertTripleArchTypeToEMachine(Triple::ArchType ArchType);
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frobtech wrote:

There is `ELFObjectFile::getArch()` going in the other direction, with pretty minimal coverage.
AFAICT the actual emission is set by each `MCELFObjectTargetWriter` subclass's constructor.
I don't really see any straightforward way to share a single source of truth there.

https://github.com/llvm/llvm-project/pull/128559