CONFIG_PCI_DRIVERS_LEGACY is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_PCI_DRIVERS_LEGACY (not configurable)

modulename: pci-legacy.ko

configname: CONFIG_PCI_DRIVERS_LEGACY

Linux Kernel Configuration
└─>Bus options (PCI, PCMCIA, EISA, ISA, TC)
└─>PCI_DRIVERS_LEGACY
In linux kernel since version 4.1 (release Date: 2015-06-21)  
source code:
arch/mips/pci/pci-legacy.c