Slightly increase performance of system::getNanoseconds() for Linux.

include/system.hpp

@@ -36,8 +36,9 @@ int getLogicalCoreCount();
 void setThreadName(const std::string& name);
 /** Returns the caller's human-readable stack trace with "\n"-separated lines. */
 std::string getStackTrace();
-/** Returns the current number of nanoseconds since the epoch with the highest precion available on the OS.
-The epoch is undefined and could be the UNIX epoch, time since boot, time since launch, etc.
+/** Returns the current number of nanoseconds since the epoch.
+The goal of this function is to give the most precise (fine-grained) time available on the OS for benchmarking purposes, while being fast to compute.
+The epoch is undefined. Do not use this function to get absolute time, as it is different on each OS.
 */
 int64_t getNanoseconds();
 /** Opens a URL, also happens to work with PDFs and folders.

src/system.cpp

@@ -225,7 +225,14 @@ int64_t getNanoseconds() {
 	int64_t nsPerTick = 1000000000LL / frequency.QuadPart;
 	int64_t time = counter.QuadPart * nsPerTick;
 	return time;
-#else
+#endif
+#if defined ARCH_LIN
+	struct timespec ts;
+	clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
+	int64_t time = int64_t(ts.tv_sec) * 1000000000LL + ts.tv_nsec;
+	return time;
+#endif
+#if defined ARCH_MAC
 	using clock = std::chrono::high_resolution_clock;
 	using time_point = std::chrono::time_point<clock>;
 	time_point now = clock::now();