Fix VulkanFunctionTable alignment

Summary:
# Problem:
If someone uses VulkanFunctionTable outside IGL and have different set of defines (not one with what IGL was compiled), loading procedure inside IGL becomes incompatible with external VulkanFunctionTable because of wrong alignment.

# Fix:
Generate alignments for optional blocks inside VulkanFunctionTable. It helps to maintain stable addresses for all functions.

Reviewed By: EricGriffith

Differential Revision: D49950763

fbshipit-source-id: 23fb488a7bdb7210ae18c946eb6e4417ad4007fd