The _SFR_MEM8() is a macro to return a byte of data of the specified address.
The _SFR_MEM8() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.
#define _SFR_MEM8(mem_addr) _MMIO_BYTE(mem_addr)
It calls _MMIO_BYTE().