gn2bp: Use different srcs based on arch

srcs of partition_allocator:partition_alloc contains x86_64 specific asm
code. This CL add hardcoded rule to support x64 arch by arch condition
for srcs.

Test: m cronet_aml_components_cronet_android_cronet
Change-Id: I45f9a57fd374fc6a6c6fa4394591904a7c739e14