[libc++][Android] Disable pbump2gig for Android

Android devices frequently don't have enough memory to run this test.

e.g. On the API 33 x86-64 emulator with 2GiB RAM, the test triggers the
OOM Killer, the device reboots, and then `adb shell` exits with status
0 and no output.

Reviewed By: danalbert, #libc, Mordante

Differential Revision: https://reviews.llvm.org/D139502