Don't assume the existence of HWCAP2_MTE.

Some ARMv8 machines fail to build with "error: use of undeclared identifier
'HWCAP2_MTE'"

According to the kernel documentation[1],

> ARMv8.5 based processors introduce the Memory Tagging Extension (MTE) feature.
>
> When CONFIG_ARM64_MTE is selected and Memory Tagging Extension is supported by
> the hardware, the kernel advertises the feature to userspace via HWCAP2_MTE.

[1] https://www.kernel.org/doc/html/latest/arm64/memory-tagging-extension.html#userspace-support

Bug: 1454438
Change-Id: I2d300a22cb13e7446d8b62428bb8e982eedeb7bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4610745
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Richard Townsend <richard.townsend@arm.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1162311}