ArmVirtPkg/PrePi: clear frame pointer in startup code

ArmVirtPkg/PrePi: clear frame pointer in startup code

When setting up the stack in the startup code and jumping into C code
for the first time, ensure that the frame pointer register is cleared
so that backtraces terminate correctly. Otherwise, output like the
below is shown when encountering an exception on a DEBUG build:

  Synchronous Exception at 0x0000000078A14780
  PC 0x000078A14780 (0x000078A00000+0x00014780) [ 0] ArmVeNorFlashDxe.dll
  PC 0x000078A10634 (0x000078A00000+0x00010634) [ 0] ArmVeNorFlashDxe.dll
  PC 0x000078A11CF0 (0x000078A00000+0x00011CF0) [ 0] ArmVeNorFlashDxe.dll
  PC 0x000078A11DD0 (0x000078A00000+0x00011DD0) [ 0] ArmVeNorFlashDxe.dll
  PC 0x00007BC9D87C (0x00007BC98000+0x0000587C) [ 1] PartitionDxe.dll
  PC 0x00007BC99B3C (0x00007BC98000+0x00001B3C) [ 1] PartitionDxe.dll
  PC 0x00007F362F50 (0x00007F359000+0x00009F50) [ 2] DxeCore.dll
  PC 0x00007BD63BB0 (0x00007BD5B000+0x00008BB0) [ 3] BdsDxe.dll
  PC 0x00007BD6EE00 (0x00007BD5B000+0x00013E00) [ 3] BdsDxe.dll
  PC 0x00007BD67C70 (0x00007BD5B000+0x0000CC70) [ 3] BdsDxe.dll
  PC 0x00007F3724F0 (0x00007F359000+0x000194F0) [ 4] DxeCore.dll
  PC 0x00004008FC30
  PC 0x000040090130
  PC 0x5800F6025800F5E1

  Recursive exception occurred while dumping the CPU state

which is rather unhelpful, given that this prevent the remaining debug
output from being printed (register dump, stack dump, fault related
system registers etc)

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>