objtool: Use '.strtab' if '.shstrtab' doesn't exist, to support ORC tables on Clang

Clang puts its section header names in the '.strtab' section instead of
'.shstrtab', which causes objtool to fail with a "can't find
.shstrtab section" warning when attempting to write ORC metadata to an
object file.

If '.shstrtab' doesn't exist, use '.strtab' instead.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/d1c1c3fe55872be433da7bc5e1860538506229ba.1531153015.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/elf.c

@@ -519,10 +519,12 @@ struct section *elf_create_section(struct elf *elf, const char *name,
 	sec->sh.sh_flags = SHF_ALLOC;
 
 
-	/* Add section name to .shstrtab */
+	/* Add section name to .shstrtab (or .strtab for Clang) */
 	shstrtab = find_section_by_name(elf, ".shstrtab");
+	if (!shstrtab)
+		shstrtab = find_section_by_name(elf, ".strtab");
 	if (!shstrtab) {
-		WARN("can't find .shstrtab section");
+		WARN("can't find .shstrtab or .strtab section");
 		return NULL;
 	}