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