pass appropriate libc path

The same as https://github.com/ruby/ruby/pull/2686, but for musl libc.
Musl is not named as libc.so.6 so the `ldd` hack implemented some lines
below does not work.

test/fiddle/helper.rb

@@ -17,6 +17,10 @@ when /android/
   end
   libc_so = File.join(libdir, "libc.so")
   libm_so = File.join(libdir, "libm.so")
+when /linux-musl/
+  Dir.glob('/lib/ld-musl-*.so.1') do |ld|
+    libc_so = libm_so = ld
+  end
 when /linux/
   libdir = '/lib'
   case RbConfig::SIZEOF['void*']