Global sort add index should use fixed memory size

[tangenta]

Enhancement

Currently, we use available memory to determine writer memory size when read-index stage starts.

tidb/pkg/ddl/backfilling_operators.go

Lines 120 to 128 in 8a10278

	memTotal, err := memory.MemTotal()
	if err != nil {
	return 0, err
	}
	memUsed, err := memory.MemUsed()
	if err != nil {
	return 0, err
	}
	memAvailable := memTotal - memUsed

This value can be different under different workloads. As a result, we may encounter the error "merge sort multi parts exceed 10000", because we cannot make sure the following formula if memsize is unstable:

partsize * 10000 >= 4000 * memsize