OvmfPkg/PlatformInitLib: Implement multi-core accept memory for TDVF

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4172

TDVF once accepts memory only by BSP. To improve the boot performance
this patch introduce the multi-core accpet memory. Multi-core means
BSP and APs work together to accept memory.

TDVF leverages mailbox to wake up APs. It is not enabled in MpInitLib
(Which requires SIPI). So multi-core accept memory cannot leverages
MpInitLib to coordinate BSP and APs to work together.

So TDVF split the accept memory into 2 phases.
- AcceptMemoryForAPsStack:
  BSP accepts a small piece of memory which is then used by APs to setup
  stack. We assign a 16KB stack for each AP. So a td-guest with 256 vCPU
  requires 255*16KB = 4080KB.
- AcceptMemory:
  After above small piece of memory is accepted, BSP commands APs to
  accept memory by sending AcceptPages command in td-mailbox. Together
  with the command and accpet-function, the APsStack address is send
  as well. APs then set the stack and jump to accept-function to accept
  memory.

AcceptMemoryForAPsStack accepts as small memory as possible and then jump
to AcceptMemory. It fully takes advantage of BSP/APs to work together.
After accept memory is done, the memory region for APsStack is not used
anymore. It can be used as other private memory. Because accept-memory
is in the very beginning of boot process and it will not impact other
phases.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

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