x86/Kconfig: Enable X86_X2APIC by default and improve help text

As many current platforms (most modern Intel CPUs and QEMU) have x2APIC
present, enable CONFIG_X86_X2APIC by default as it gives performance
and functionality benefits. Additionally, if the BIOS has already
switched APIC to x2APIC mode, but CONFIG_X86_X2APIC is disabled, the
kernel will panic in arch/x86/kernel/apic/apic.c .

Also improve the help text, which was confusing and really did not
describe what the feature is about.

Help text references and discussion:

Both Intel [1] and AMD [3] spell the name as "x2APIC", not "x2apic".

"It allows faster access to the local APIC"
        [2], chapter 2.1, page 15:
        "More efficient MSR interface to access APIC registers."

"x2APIC was introduced in Intel CPUs around 2008":
        I was unable to find specific information which Intel CPUs
        support x2APIC. Wikipedia claims it was "introduced with the
        Nehalem microarchitecture in November 2008", but I was not able
        to confirm this independently. At least some Nehalem CPUs do not
        support x2APIC [1].

        The documentation [2] is dated June 2008. Linux kernel also
        introduced x2APIC support in 2008, so the year seems to be
        right.

"and in AMD EPYC CPUs in 2019":
        [3], page 15:
        "AMD introduced an x2APIC in our EPYC 7002 Series processors for
        the first time."

"It is also frequently emulated in virtual machines, even when the host
CPU does not support it."
        [1]

"If this configuration option is disabled, the kernel will not boot on
some platforms that have x2APIC enabled."
        According to some BIOS documentation [4], the x2APIC may be
        "disabled", "enabled", or "force enabled" on this system.
        I think that "enabled" means "made available to the operating
        system, but not already turned on" and "force enabled" means
        "already switched to x2APIC mode when the OS boots". Only in the
        latter mode a kernel without CONFIG_X86_X2APIC will panic in
        validate_x2apic() in arch/x86/kernel/apic/apic.c .

	QEMU 4.2.1 and my Intel HP laptop (bought in 2019) use the
	"enabled" mode and the kernel does not panic.

[1] "Re: [Qemu-devel] [Question] why x2apic's set by default without host sup"
        https://lists.gnu.org/archive/html/qemu-devel/2013-07/msg03527.html

[2] Intel® 64 Architecture x2APIC Specification,
        ( https://www.naic.edu/~phil/software/intel/318148.pdf )

[3] Workload Tuning Guide for AMD EPYC ™ 7002 Series Processor Based
        Servers Application Note,
        https://developer.amd.com/wp-content/resources/56745_0.80.pdf

[4] UEFI System Utilities and Shell Command Mobile Help for HPE ProLiant
        Gen10, ProLiant Gen10 Plus Servers and HPE Synergy:
        Enabling or disabling Processor x2APIC Support
        https://techlibrary.hpe.com/docs/iss/proliant-gen10-uefi/s_enable_disable_x2APIC_support.html

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250321-x86_x2apic-v3-1-b0cbaa6fa338@ixit.cz

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