8387407: AARCH: use CNTVCTSS_EL0 as fast unordered timestamp source for JFR

[bulasevich]

AArch64 processors with FEAT_ECV (Armv8.6+) expose a self-synchronized counter via the CNTVCTSS_EL0 register, which can serve as a low-overhead timestamp source for JFR. Added cntvctss_aarch64.cpp/hpp that reads CNTVCTSS_EL0 and mirrors the x86 Rdtsc interface. Reduces JFR event commit latency on AArch64 with FEAT_ECV, replacing clock_gettime with a direct register read.

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• JDK-8387407: AARCH: use CNTVCTSS_EL0 as fast unordered timestamp source for JFR (Enhancement - P3)

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