PR #1644: unscaledcycleclock: remove RISC-V support

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1644

Starting with Linux 6.6 [1], RDCYCLE is a privileged instruction on RISC-V and can't be used directly from userland. There is a sysctl option to change that as a transition period, but it will eventually disappear.

The RDTIME instruction is another less accurate alternative, however its frequency varies from board to board, and there is currently now way to get its frequency from userland [2].

Therefore this patch just removes the code for unscaledcycleclock on RISC-V. Without processor specific implementation, abseil relies on std::chrono::steady_clock::now().time_since_epoch() which is basically a wrapper around clock_gettime (CLOCK_MONOTONIC), which in turns use __vdso_clock_gettime(). On RISC-V this VDSO is just a wrapper around RDTIME correctly scaled to use nanoseconds units.

This fixes the testsuite on riscv64, tested on a VisionFive 2 board.

[1] cc4c07c89aada16229084eeb93895c95b7eabaa3">https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3
[2] https://github.com/abseil/abseil-cpp/pull/1631
Merge 43356a2548cfde76e164d446cb69004b488c6a71 into 76f8011beabdaee872b5fde7546e02407b220cb1

Merging this change closes #1644

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1644 from aurel32:rv64-no-unscaledcycleclock 43356a2548cfde76e164d446cb69004b488c6a71
PiperOrigin-RevId: 618286262
Change-Id: Ie4120a727e7d0bb185df6e06ea145c780ebe6652

0 files changed, 0 insertions, 0 deletions