unscaledcycleclock: remove RISC-V support

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] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3
[2] abseil#1631

absl/base/internal/unscaledcycleclock.cc

@@ -121,18 +121,6 @@ double UnscaledCycleClock::Frequency() {
   return aarch64_timer_frequency;
 }
 
-#elif defined(__riscv)
-
-int64_t UnscaledCycleClock::Now() {
-  int64_t virtual_timer_value;
-  asm volatile("rdcycle %0" : "=r"(virtual_timer_value));
-  return virtual_timer_value;
-}
-
-double UnscaledCycleClock::Frequency() {
-  return base_internal::NominalCPUFrequency();
-}
-
 #elif defined(_M_IX86) || defined(_M_X64)
 
 #pragma intrinsic(__rdtsc)

absl/base/internal/unscaledcycleclock_config.h

@@ -21,8 +21,8 @@
 
 // The following platforms have an implementation of a hardware counter.
 #if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) || \
-    defined(__powerpc__) || defined(__ppc__) || defined(__riscv) ||     \
-    defined(_M_IX86) || (defined(_M_X64) && !defined(_M_ARM64EC))
+    defined(__powerpc__) || defined(__ppc__) || defined(_M_IX86) ||     \
+    (defined(_M_X64) && !defined(_M_ARM64EC))
 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 1
 #else
 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 0
@@ -53,7 +53,7 @@
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 // This macro can be used to test if UnscaledCycleClock::Frequency()
 // is NominalCPUFrequency() on a particular platform.
-#if (defined(__i386__) || defined(__x86_64__) || defined(__riscv) || \
+#if (defined(__i386__) || defined(__x86_64__) || \
      defined(_M_IX86) || defined(_M_X64))
 #define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
 #endif