Before this commit, there were contradictory descriptions about size of EFER
register.
Line 113 says the size is 8 bytes.
Line 129 says the size is 4 bytes.
As a result, when GDB is debugging an OS running on QEMU, the GDB cannot
read 'g' packets correctly. This 'g' packet transmits values of each
registers of machine emulated by QEMU to GDB. QEMU, the packet sender,
assign 4 bytes for EFER in 'g' packet based on the line 113.
GDB, the packet receiver, extract 8 bytes for EFER in 'g' packet based on
the line 129. Therefore, all registers located behind EFER in 'g' packet
has been shifted 4 bytes in GDB.
After this commit, GDB can read 'g' packets correctly.