AArch64: correct Cortex-X4 MIDR

AArch64: correct Cortex-X4 MIDR

The Parts Num field for the MIDR for Cortex-X4 is wrong.  It's currently the
parts number for a Cortex-A720 (which does have the right number).

The correct number can be found in the Cortex-X4 Technical Reference Manual [1]
on page 382 in Issue Number 5.

[1] https://developer.arm.com/documentation/102484/latest/

gcc/ChangeLog:

	* config/aarch64/aarch64-cores.def (AARCH64_CORE): Fix cortex-x4 parts
	num.