[rubygems/rubygems] Fix Bundler::Digest#sha1 on big-endian systems

As noticed by @nobu https://github.com/rubygems/rubygems/pull/4989#discussion_r735674633

From wikipedia: https://en.wikipedia.org/wiki/SHA-1#SHA-1_pseudocode

> append ml, the original message length in bits, as a 64-bit big-endian integer.

`Q` is native endian, so little-endian on most modern hardware.
The original code from RubyDigest reverses the bytes:
d15f906caf/lib/ruby_digest.rb (L521)

But that makes the code non-portable, the correct way is to directly ask
for a big-endian representation.

https://github.com/rubygems/rubygems/commit/ba2be01ea4

lib/bundler/digest.rb

@@ -59,7 +59,7 @@ module Bundler
         size   = string.bytesize * 8
         buffer = string.bytes << 128
         buffer << 0 while buffer.size % 64 != 56
-        [size].pack("Q").bytes.reverse_each {|b| buffer << b }
+        buffer.concat([size].pack("Q>").bytes)
         buffer.each_slice(64, &block)
       end