Use VDSO for dr_get_microseconds

Today, (on unix) dr_get_microseconds executes a gettimeofday syscall directly. To avoid overhead during this call for clients which need the timestamp very often (e.g. drcachesim offline trace support) we should arrange for accessing this information via the VDSO for unix, or KUSER_SHARED_DATA for windows.

xref comment in drcachesim's offline trace support: https://github.com/DynamoRIO/dynamorio/blob/dc7e314b9957d46208f5b5aa5fe85b5000ae7d02/clients/drcachesim/tracer/instru_offline.cpp#L283