executor: sys/linux: implement SYZOS_API_NESTED_VMRESUME

Provide the SYZOS API command to resume L2 execution after a VM exit,
using VMRESUME on Intel and VMRUN on AMD.

For testing purpose, implement basic handling of the INVD instruction:
 - enable INVD interception on AMD (set all bits in VMCB 00Ch);
 - map EXIT_REASON_INVD and VMEXIT_INVD into SYZOS_NESTED_EXIT_REASON_INVD;
 - advance L2 RIP to skip to the next instruction.

While at it, perform minor refactorings of L2 exit reason handling.

sys/linux/test/amd64-syz_kvm_nested_vmresume tests the new command by
executing two instructions, INVD and HLT, in the nested VM.

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