Tidy and document the `inline`/`__inline`/`__inline__` situation.

We can (and should) use `inline` in C++ but not in C headers that
need to support ANSI/C89/C90. `__inline__` works everywhere, but
traditionally we've used the BSD `__inline` in public headers.

Change-Id: I920e0a1f5fcd01cf8ddd89240ef66b25c50378d7
diff --git a/benchmarks/util.h b/benchmarks/util.h
index 99eed5f..347dc35 100644
--- a/benchmarks/util.h
+++ b/benchmarks/util.h
@@ -71,7 +71,7 @@
 
 bool LockToCPU(int cpu_to_lock);
 
-static __inline __attribute__ ((__always_inline__)) void MakeAllocationResident(
+static inline __attribute__((__always_inline__)) void MakeAllocationResident(
     void* ptr, size_t nbytes, int pagesize) {
   uint8_t* data = reinterpret_cast<uint8_t*>(ptr);
   for (size_t i = 0; i < nbytes; i += pagesize) {