Fix #4866, msfvenom not properly handling platform & arch

This fixes #4866, an issue with msfvenom not properly handling special
cases with generic payloads. So the story behind this fix is that
we have these two problems:

Problem 1: The current payload selection design relies on the payload
module in order to set the platform and arch. Almost all MSF payloads
contain a default platform and arch, however, the bind and reverse
generic payloads don't.

Problem 2: By default, Msf::Payload::Generic also explicitly sets the
PLATFORM and ARCH datastore options to nil. So there is no way the
payload generator can figure out what platform and arch to use.

As a result of these problems, msfvenom will actually end up getting
a Msf::Module::Platform as the default platform, which doesn't
actually represent any valid platform we can use (such as
Msf::Module::Platform::Windows). And the first item of ARCH_ALL for
the arch.

In addition, msfvenom has these two arguments that the user can use:
--platform and --arch. In most cases, these arguments are used more
like checks than actually setting anything. Because remember:
Framework's payload selector retreives the platform & arch from the
module (trusted), not the user input (untrusted). But from the user's
perspective it's impossible to know this.

After experimenting different ways to fix this, I came up with this
patch. It feels sort of more like a hack than a real fix, but as
far as I can tell, this is the best you can get unless you want to
redesign generic payload selection.

lib/msf/core/payload_generator.rb

@@ -144,8 +144,10 @@ module Msf
       if arch.blank?
         @arch = mod.arch.first
         cli_print "No Arch selected, selecting Arch: #{arch} from the payload"
+        datastore['ARCH'] = arch if mod.kind_of?(Msf::Payload::Generic)
         return mod.arch.first
       elsif mod.arch.include? arch
+        datastore['ARCH'] = arch if mod.kind_of?(Msf::Payload::Generic)
         return arch
       else
         return nil
@@ -157,7 +159,10 @@ module Msf
     # @param mod [Msf::Payload] The module class to choose a platform for
     # @return [Msf::Module::PlatformList] The selected platform list
     def choose_platform(mod)
+      # By default, platform_list will at least return Msf::Module::Platform
+      # if there is absolutely no pre-configured platform info at all
       chosen_platform = platform_list
+
       if chosen_platform.platforms.empty?
         chosen_platform = mod.platform
         cli_print "No platform was selected, choosing #{chosen_platform.platforms.first} from the payload"
@@ -165,6 +170,17 @@ module Msf
       elsif (chosen_platform & mod.platform).empty?
         chosen_platform = Msf::Module::PlatformList.new
       end
+
+      begin
+        platform_object = Msf::Module::Platform.find_platform(platform)
+      rescue ArgumentError
+        platform_object = nil
+      end
+
+      if mod.kind_of?(Msf::Payload::Generic) && mod.send(:module_info)['Platform'].empty? && platform_object
+        datastore['PLATFORM'] = platform
+      end
+
       chosen_platform
     end