StdLib: reinstate the use of va_arg() to handle long double arguments in vfscanf.

StdLib: reinstate the use of va_arg() to handle long double arguments in vfscanf.
NOTE: Replaces the previous version which was a file from the wrong project.

Applies the patch, submitted by Olivier Martin, to use va_arg for long double.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Daryl McDaniel <daryl.mcdaniel@intel.com>
Includes some cosmetic changes to enhance readability.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Daryl McDaniel <daryl.mcdaniel@intel.com>
Reviewed-by: Olivier Martin <Olivier.Martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15859 6f19259b-4bc3-4df7-8a09-765794883524