Add retry loop around GetMemoryMap call to fix fragmentation bug

The call to BS->AllocatePages can cause the memory map to become framented,
causing BS->GetMemoryMap to return EFI_BUFFER_TOO_SMALL more than once. For
example this can happen on the MinnowBoard Turbot, causing the boot to stop
with an error. Avoid this by calling GetMemoryMap in a loop.

Reviewed by: imp, tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D19341