Silence llvm-nm "not recognized" noise on LTO bitcode

Under -flto the compiler emits raw LLVM bitcode objects, but the
configured nm carries --no-llvm-bc (added so it ignores Rust's
newer-version bitcode) and rejects them with "not recognized", flooding
the clang-22 LTO build log. leaked-globals now drops those objects, still
checking the linked library and any ELF objects, and the symbol-prefix
probe in configure silences nm's stderr.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

configure.ac

@@ -4163,7 +4163,7 @@ AC_SUBST(JIT_CARGO_SUPPORT)dnl "no" or the cargo profile of the rust code
 [begin]_group "build section" && {
 AC_CACHE_CHECK([for prefix of external symbols], rb_cv_symbol_prefix, [
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[extern void conftest_external(void) {}]], [[]])],[
-	rb_cv_symbol_prefix=`$NM conftest.$ac_objext |
+	rb_cv_symbol_prefix=`$NM conftest.$ac_objext 2>/dev/null |
 			     sed -n ['/.*T[ 	]\([^ 	]*\)conftest_external.*/!d;s//\1/p;q']`
 	],
 	[rb_cv_symbol_prefix=''])

tool/leaked-globals

@@ -60,6 +60,23 @@ end
 # extern, so we allow the Rust one.
 REPLACE.push("rust_eh_personality") if RUBY_PLATFORM.include?("darwin")
 
+# Under LTO the compiler emits raw LLVM bitcode objects (magic "BC\xC0\xDE", or
+# the bitcode wrapper magic).  nm carries --no-llvm-bc (added in configure so it
+# ignores Rust's newer-version bitcode) and so cannot read them, only emitting
+# "not recognized" noise.  Drop them; leaked symbols are still checked on the
+# linked library and any non-bitcode objects.
+if defined?(NM) and NM.include?("--no-llvm-bc")
+  bitcode = 0
+  ARGV.reject! do |n|
+    next false unless File.file?(n)
+    case File.binread(n, 4)&.bytes
+    when [0x42, 0x43, 0xC0, 0xDE], [0xDE, 0xC0, 0x17, 0x0B]
+      bitcode += 1
+    end
+  end
+  puts "Skip #{col.skip("#{bitcode} LLVM bitcode object(s)")}" if bitcode > 0
+end
+
 print "Checking leaked global symbols..."
 STDOUT.flush
 if soext
@@ -105,7 +122,7 @@ Pipe.new(NM + ARGV).each do |line|
   puts col.fail("leaked") if count.zero?
   count += 1
   puts "  #{n}"
-end
+end unless ARGV.empty?
 case count
 when 0
   puts col.pass("none")