PR #2147: Fix FreeBSD/PowerPC* issues

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/2147

Fixes two issues on FreeBSD PowerPC architectures:

1. UnscaledCycleClock::Frequency():
   The `kern.timecounter.tc.timebase.frequency` sysctl is registered in the
   FreeBSD kernel as CTLTYPE_U64 (uint64_t). Previously, sysctlbyname() read
   directly into a `double` variable without type conversion, causing the raw
   integer bits to be reinterpreted as a double.

2. StacktracePowerPCGetLRPtr():
   When compiler ABI macros are absent, the unwinder falls back to OS checks.
   Previously, __FreeBSD__ was grouped with __APPLE__ to read the saved Link
   Register at sp[2]. While sp[2] (offset 16) is correct for 64-bit ELF / AIX /
   Darwin, the 32-bit PowerPC SysV ELF ABI saves the LR at sp[1] (offset 4).
   Fix by keying the fallback on __PPC64__ for 64-bit (sp[2]) versus __linux__
   and __FreeBSD__ for 32-bit (sp[1]).

Merge 8f4431186a50594b7dced86835d4b0a5be708dd6 into 2adadbc666a2da417194f73694346f41969e0777

Merging this change closes #2147

PiperOrigin-RevId: 976301714
Change-Id: I584c7231e5071cde514a02c082a49f829a414963

absl/base/internal/unscaledcycleclock.cc

diff --git a/absl/base/internal/unscaledcycleclock.cc b/absl/base/internal/unscaledcycleclock.cc
index 9e93180..28fe66e 100644
--- a/absl/base/internal/unscaledcycleclock.cc
+++ b/absl/base/internal/unscaledcycleclock.cc
@@ -99,9 +99,12 @@
   static once_flag init_timebase_frequency_once;
   static double timebase_frequency = 0.0;
   base_internal::LowLevelCallOnce(&init_timebase_frequency_once, [&]() {
-    size_t length = sizeof(timebase_frequency);
-    sysctlbyname("kern.timecounter.tc.timebase.frequency", &timebase_frequency,
-                 &length, nullptr, 0);
+    uint64_t freq = 0;
+    size_t length = sizeof(freq);
+    if (sysctlbyname("kern.timecounter.tc.timebase.frequency", &freq, &length,
+                     nullptr, 0) == 0) {
+      timebase_frequency = static_cast<double>(freq);
+    }
   });
   return timebase_frequency;
 #else

absl/debugging/internal/stacktrace_powerpc-inl.inc

diff --git a/absl/debugging/internal/stacktrace_powerpc-inl.inc b/absl/debugging/internal/stacktrace_powerpc-inl.inc
index f23c29c..a20a715 100644
--- a/absl/debugging/internal/stacktrace_powerpc-inl.inc
+++ b/absl/debugging/internal/stacktrace_powerpc-inl.inc
@@ -51,11 +51,10 @@
   return (sp + 2);
 #elif defined(_CALL_SYSV)
   return (sp + 1);
-#elif defined(__APPLE__) || defined(__FreeBSD__) || \
-    (defined(__linux__) && defined(__PPC64__))
+#elif defined(__APPLE__) || defined(__PPC64__)
   // This check is in case the compiler doesn't define _CALL_AIX/etc.
   return (sp + 2);
-#elif defined(__linux)
+#elif defined(__linux__) || defined(__FreeBSD__)
   // This check is in case the compiler doesn't define _CALL_SYSV.
   return (sp + 1);
 #else