[android] change shortname for Android arch definitions

[andrurogerz]

Overview

Change the short name for the Android i686 arch to x86 and the x86_64 arch to x64. These names match the names of the containing variables, $AndroidX86 and $AndroidX64 (variable names are not case sensitive).

Background

Without this change, any build.ps1 build on Windows that includes the x86_64 Android SDK fails with an error like so:

Error: Cannot find a variable with the name 'Androidx86_64'.
    at <ScriptBlock>, S:\SourceCache\swift\utils\build.ps1: line 1873
    at Isolate-EnvVars, S:\SourceCache\swift\utils\build.ps1: line 581
    at Build-Foundation, S:\SourceCache\swift\utils\build.ps1: line 1869
    at Invoke-BuildStep, S:\SourceCache\swift\utils\build.ps1: line 416
    at <ScriptBlock>, S:\SourceCache\swift\utils\build.ps1: line 2791
  From System.Management.Automation.ItemNotFoundException: Cannot find a variable with the name 'Androidx86_64'.

And for the i686 arch:

Error: Cannot find a variable with the name 'Androidi686'.
    at <ScriptBlock>, S:\SourceCache\swift\utils\build.ps1: line 1873
    at Isolate-EnvVars, S:\SourceCache\swift\utils\build.ps1: line 581
    at Build-Foundation, S:\SourceCache\swift\utils\build.ps1: line 1869
    at Invoke-BuildStep, S:\SourceCache\swift\utils\build.ps1: line 416
    at <ScriptBlock>, S:\SourceCache\swift\utils\build.ps1: line 2791
  From System.Management.Automation.ItemNotFoundException: Cannot find a variable with the name 'Androidi686'.

The build succeeds if only aarch64 and/or armv7 are specified, since their short names are already correct.