Drop SHF_COMPRESSED when creating a relocatable file

We always uncompress compressed sections when copying, so we should
not propagate the flag.

Fixes #1195

elf/output-chunks.cc

@@ -864,7 +864,7 @@ OutputSection<E>::OutputSection(Context<E> &ctx, std::string_view name,
                                 u32 type, u64 flags) {
   this->name = name;
   this->shdr.sh_type = type;
-  this->shdr.sh_flags = flags & ~SHF_MERGE & ~SHF_STRINGS;
+  this->shdr.sh_flags = flags & ~SHF_MERGE & ~SHF_STRINGS & ~SHF_COMPRESSED;
 
   if (auto it = ctx.arg.section_align.find(name);
       it != ctx.arg.section_align.end())

test/elf/relocatable-compressed-debug-info.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+# OneTBB isn't tsan-clean
+nm mold | grep -q '__tsan_init' && skip
+
+cat <<EOF | $GCC -c -o $t/a.o -xc - -g3 -gz
+#include <stdio.h>
+void hello() { printf("Hello world\n"); }
+EOF
+
+cat <<EOF | $GCC -c -o $t/b.o -xc - -g3 -gz
+void hello();
+int main() { hello(); }
+EOF
+
+./mold --relocatable -o $t/c.o $t/a.o $t/b.o
+$CC -B. -o $t/exe $t/c.o
+$QEMU $t/exe