ArmPkg/DefaultExceptionHandlerLib: trim module name in backtrace output

ArmPkg/DefaultExceptionHandlerLib: trim module name in backtrace output

As reported by Vishal, the new backtrace output would be more useful if
it did not contain the full absolute path of each module in the list.
So strip off everything up to the last forward slash or backslash in the
string.

Example output:

  IRQ Exception at 0x000000005EF110E0
  DxeCore.dll loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EF121F0) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EF1289C) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEFB6B4) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEFAA44) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEFB450) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEF938C) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEF8D04) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEFA8E8) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEF3C14) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEF3E48) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EF0C838) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEEF70C) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEEE93C) loaded at 0x000000005EEED000
  called from DxeCore.dll (0x000000005EEEE024) loaded at 0x000000005EEED000

Suggested-by: "Oliyil Kunnil, Vishal" <vishalo@qti.qualcomm.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>