[Test][JITLink] Save rbx in ExecutionEngine/JITLink/x86-64/ELF_vtune.s (#86472)

The callee should preserve rbx according to the calling convention, but
it is not in the test case `ExecutionEngine/JITLink/x86-64/ELF_vtune.s`.
Not preserving the rbx register may result in some random error to the
caller function. This patch adds the missing command to preserve the
rbx.