Updated vex submodule pointer to master

[EvelynVusky]

• Updated the vex submodule pointer to point to most recent commit on master: https://github.com/angr/vex/commit/8c3b99ef0c842d926e11b93ea0ddb1453e3f61b4
• The most recent commit includes RISCV support for non-FP CSR operations and instructions

[ltfish]

Thanks!