BaseTools: Fix the bug to parse the short varname in map file

BaseTools: Fix the bug to parse the short varname in map file

current in the map file, there have two ways for var to save its offset,
if the varname is short, then the offset will in the same line with
varname, otherwise, it saved in the next line.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>