add back an alias for check_top_bit (#53523)

Used in some packages (e.g. rfourquet/BitIntegers.jl)
Renamed in #53166

(cherry picked from commit 24aaf00)

base/boot.jl

@@ -969,6 +969,7 @@ arrayset(inbounds::Bool, A::Array{T}, x::Any, i::Int...) where {T} = Main.Base.s
 arraysize(a::Array) = a.size
 arraysize(a::Array, i::Int) = sle_int(i, nfields(a.size)) ? getfield(a.size, i) : 1
 export arrayref, arrayset, arraysize, const_arrayref
+const check_top_bit = check_sign_bit
 
 # For convenience
 EnterNode(old::EnterNode, new_dest::Int) = isdefined(old, :scope) ?