Rename gpu_mem.o to gpuMem.o

Underscore character may cause bpf prog/map naming collision. For
example, x.o with map y_z and x_y.o with map z both result in x_y_z
prog/map name, which should be prevented during compile-time.

aosp/2147825 will prohibit underscore character in bpf source name
(source name derives the obj name). Existing bpf modules with underscore
characters in source name need to be updated accordingly.

Bug: 236706995
Test: adb root; adb shell ls -l /sys/fs/bpf/ | grep gpuMem
Change-Id: I34e7a786d88fec1bd4dc0e3fe322fd46b3876bc5