Fix file-size stats on OSX

Kernel makefiles that the stat on the system is GNU stat. GNU stat uses the "-c" option to specify format.
Darwin stat uses the "-f" option to specify format. This discrepency will cause kernel build breaks.
On my system, I symlink stat to GNU stat. This causes the "get-file-size" define
to fail.
The fix for this is to detect "gstat", aka GNU stat, and use that appropriately.

Change-Id: I987c155b7dc3ff14ffe6da40edf834ca34b7df75

Fix up the get-file-size function that was broken due to the prior commit that made it utilize gnu stat (gstat)

Change-Id: I24bba2bfcb509ad1ad76d2260eedd685ba45c393