ShellPkg: Fix GCC build fail and code refine.

ShellPkg: Fix GCC build fail and code refine.

1. Fix GCC build fail.
2. It's not correct to cast away constness to allow TrimSpaces() to modify 'commandline'.
   This patch makes a copy of 'commandLine' and work with that in the remainder of the function.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18500 6f19259b-4bc3-4df7-8a09-765794883524