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Don't expose vaddr_t or register_t to userland. The gdb configure script
needs this so it can detect struct lwp correctly.

RISC-V support that works on QEMU with a single hart.

Thanks for Simon Burge for plic(4).

Optimize the exception handle a little bit more.

Add a md_tp member to mdlwp so that the exception handler can temporarily
store the user's thread pointer before saving it in the trapframe.

New files for Userland support of UCB RISC-V (both 32-bit and 64-bit)