[Mach-O] arm64e.x1 is mislabeled as arm64 in UI

[bdash]

Version and Platform (required):

• Binary Ninja Version: 6.1.10653-dev Ultimate, 30f6d3a0
• OS: macos
• OS Version: 26.6.2
• CPU Architecture: arm64

Bug Description:
macOS 27 introduces a new arm64e.x1 CPU slice in many binaries that ship with the OS. This is a slice that is otherwise the same as arm64e, but assumes a minimum CPU baseline of Apple M6. When opening a binary containing an arm64e.x1 slice, it is reported as being a plain arm64 slice due to how our Mach-O code handles fallback when facing an unknown CPU subtype.

I expect this may result in behavioral problems if a binary contains both arm64 and arm64e.x1 slices, but I didn't find such a binary to test with.

arm64e.x1 has these aarch64 features available without any runtime detection:

#define __ARM_ARCH_8_6__ 1
#define __ARM_ARCH_8_7__ 1
#define __ARM_BF16_FORMAT_ALTERNATIVE 1
#define __ARM_FEATURE_BF16 1
#define __ARM_FEATURE_BF16_SCALAR_ARITHMETIC 1
#define __ARM_FEATURE_BF16_VECTOR_ARITHMETIC 1
#define __ARM_FEATURE_CSSC 1
#define __ARM_FEATURE_FP8 1
#define __ARM_FEATURE_LOCALLY_STREAMING 1
#define __ARM_FEATURE_MATMUL_INT8 1
#define __ARM_FEATURE_MEMORY_TAGGING 1
#define __ARM_FEATURE_PAUTH_LR 1
#define __ARM_FEATURE_SME 1
#define __ARM_FEATURE_SME2 1
#define __ARM_FEATURE_SME2p1 1
#define __ARM_FEATURE_SME_B16B16 1
#define __ARM_FEATURE_SME_F16F16 1
#define __ARM_FEATURE_SME_F8F16 1
#define __ARM_FEATURE_SME_F8F32 1
#define __ARM_FEATURE_SVE_B16B16 1

Steps To Reproduce:
Open /bin/ls from a macOS 27 system and look at the list of architectures in the Select Architecture dialog.