Add benchmarks for heap size retrieval

Add benchmarks for mallinfo, and for retrieving RSS from
/proc/self/statm, since we're considering using these for GC
triggering.

Add some static linkage specifiers, after running into a build
problem due to a spurious conflict.

Bug: 111447610

Test: Ran benchmarks
Change-Id: Ie50d512294993882728c63ce51ec507590257d80
diff --git a/benchmarks/Android.bp b/benchmarks/Android.bp
index bf8b10d..722395d 100644
--- a/benchmarks/Android.bp
+++ b/benchmarks/Android.bp
@@ -28,6 +28,7 @@
     srcs: [
         "bionic_benchmarks.cpp",
         "atomic_benchmark.cpp",
+        "get_heap_size_benchmark.cpp",
         "inttypes_benchmark.cpp",
         "malloc_benchmark.cpp",
         "math_benchmark.cpp",