Make grisu thread-safe again

[simonbyrne]

Digit printing on the slow path was still not thread-safe after #26562, e.g. with JULIA_NUM_THREADS=4 I get errors with the following:

using Base.Threads
V = fill(2.99965547519468e17, 100)
function test_without_mutex(V)
    @threads for v in V
        y = "$v"
        @assert v == parse(Float64, y)
    end
end
test_without_mutex(V)