BaseTools: Move gPlatformFinalPcd to Datapipe and optimize size

BaseTools: Move gPlatformFinalPcd to Datapipe and optimize size

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3828

This is a bugfix of
bf9230a9f3dde065c3c8b4175ccd32e44e8f0362.

1.In the current code, gPlatformFinalPcd will save all PCDs used at
whole compile process, which wastes runtime memory and is unnecessary.

This patch makes gPlatformFinalPcd save only the PCDes which are
assigned in the DSC file, and the PCD that has not been assigned will
use the default value in DEC.

2.During the compilation process, gPlatformFinalPcd may be lost, and
the current code cannot selectively assign PCD in DSC by specifying ARCH.

This patch moves gPlatformFinalPcd into datapipe and modifies the
assignment logicto fix this.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: yi1 li <yi1.li@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>