system: improve handling around GetTotalRAM()

This patch achieves two things:
1. Fix unused variable warning (https://github.com/bitcoin/bitcoin/pull/33333#discussion_r2362493046)
2. Enable GetTotalRAM() on other platforms where it was tested to work.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

Github-Pull: #33435
Rebased-From: 337a6e7386

src/common/system.cpp

@@ -113,10 +113,15 @@ int GetNumCores()
 
 std::optional<size_t> GetTotalRAM()
 {
-    auto clamp{[](uint64_t v) { return size_t(std::min(v, uint64_t{std::numeric_limits<size_t>::max()})); }};
+    [[maybe_unused]] auto clamp{[](uint64_t v) { return size_t(std::min(v, uint64_t{std::numeric_limits<size_t>::max()})); }};
 #ifdef WIN32
     if (MEMORYSTATUSEX m{}; (m.dwLength = sizeof(m), GlobalMemoryStatusEx(&m))) return clamp(m.ullTotalPhys);
-#elif defined(__linux__) || defined(__APPLE__)
+#elif defined(__APPLE__) || \
+      defined(__FreeBSD__) || \
+      defined(__NetBSD__) || \
+      defined(__OpenBSD__) || \
+      defined(__illumos__) || \
+      defined(__linux__)
     if (long p{sysconf(_SC_PHYS_PAGES)}, s{sysconf(_SC_PAGESIZE)}; p > 0 && s > 0) return clamp(1ULL * p * s);
 #endif
     return std::nullopt;