PE: Add fast path for base pointers (RBP with offset)

[ishitatsuyuki]

Pretty frequently compilers will establish a frame pointer when it's beneficial for reducing rsp manipulation. MSVC is additionally known to set RBPs to a constant offset from what is known as a "frame pointer". This will also happen with Clang if both stack realignment and alloca is involved.

We should have a new unwind rule, generalizing frame pointers, defined by:

    /// (sp, bp) = (bp + 8x, *(bp + 8y))
    UseBasePointer {
        sp_offset_from_bp_by_8: u16,
        bp_storage_offset_from_bp_by_8: i16,
    },

When x=2, y=0 this is equivalent to a frame pointer.