[llvm] [CMake] Add -Wl, -z, pack-relative-relocs if supported (PR #208217)

[Alexis Engelke via llvm-commits]

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@@ -1153,6 +1153,25 @@ if (LLVM_USE_SPLIT_DWARF AND
   endif()
 endif()
 
+# For PIC builds, enable RELR if supported by the linker and loader.
+# glibc supports RELR since 2.36. musl supports RELR since 1.2.4, but there's
+# no easy way to detect or feature-test this.
+if (LLVM_ENABLE_PIC AND LLVM_USING_GLIBC)
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aengelke wrote:

> Does LLVM_USING_GLIBC exclude musl?

musl doesn't define `__GLIBC__`. (check: `check_symbol_exists(__GLIBC__ stdio.h LLVM_USING_GLIBC)`)

I never did any productive work on a FreeBSD system, so I don't feel comfortable with doing changes there. Interested folks can do this as follow up work.

https://github.com/llvm/llvm-project/pull/208217