[BOLT] Handle ifuncs trampolines for aarch64

The aarch64 uses the trampolines located in .iplt section, which
contains plt-like trampolines on the value stored in .got. In this case
we don't have JUMP_SLOT relocation, but we have a symbol that belongs to
ifunc trampoline, so use it and set set plt symbol for such functions.

Vladislav Khmelevsky,
Advanced Software Technology Lab, Huawei

Differential Revision: https://reviews.llvm.org/D120850