ShellPkg/dmem: Only dump sizeof (EFI_SYSTEM_TABLE) bytes for gST

ShellPkg/dmem: Only dump sizeof (EFI_SYSTEM_TABLE) bytes for gST

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1236

When "dmem" runs without additional arguments, it dumps the memory
content of EFI_SYSTEM_TABLE. But today's implementation dumps 512
bytes. It's not correct because sizeof (EFI_SYSTEM_TABLE) is less
than 512, the 512-read causes page fault exception in a heap-guard
enabled environment.

The patch changes the implementation to only dump
sizeof (EFI_SYSTEM_TABLE) bytes for gST.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Jim Dailey <jim_dailey@.com>