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Fix building gio/tests/test_resources.o with LLVM ld

Alexander Richardson requested to merge arichardson/glib:build-with-llvm-toolchain into main

Unlike GNU ld which has a default target architecture, ld.lld is always a cross-linker and has the same behaviour for all targets. If you don't tell ld.lld what the target architecture is it can't infer the right ELF flags for the resulting object file.

$ ~/cheri/output/sdk/bin/ld -r -b binary gio/tests/test5.gresource -o gio/tests/test_resources.o -v
LLD 14.0.0 (compatible with GNU linkers)
ld: error: target emulation unknown: -m or at least one .o file required

As you can see from the error message it can't infer the target architecture (you need a least one valid .o file or the -m flag). If you use the compiler instead of directly invoking the linker it will pass the appropriate flags:

$ ~/cheri/output/sdk/bin/clang -r -Wl,-b,binary gio/tests/test5.gresource -o gio/tests/test_resources.o -v
clang version 14.0.0 (https://github.com/CTSRD-CHERI/llvm-project.git ff66b683475fc44355b2010dbcbe1202d785e6f8)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/alexrichardson/cheri/output/sdk/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/12
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/12
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/home/alexrichardson/cheri/output/sdk/bin/ld" --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o gio/tests/test_resources.o -L/usr/lib/gcc/x86_64-linux-gnu/12 -L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib64 -L/home/alexrichardson/cheri/output/sdk/bin/../lib -L/lib -L/usr/lib -r -b binary gio/tests/test5.gresource
❯ file gio/tests/test_resources.o
gio/tests/test_resources.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

This works for most architectures, but ones that need additional metadata sections to encode the used ABI, etc. will require a different approach using .incbin. However, that is a change for another MR.

Partially fixes: #2720 (closed)

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