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

[Fangrui Song 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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MaskRay wrote:

Newer FreeBSD also support RELR https://reviews.freebsd.org/D32524  @emaste 

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