rt: glob, globfree dummy function for android

transitional patch to resolve compile/link failure on android

after #6161 landed, I've encountered below errors since android does not support glob in libc.

/opt/ndk_standalone/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/yichoi/rust_work/build/x86_64-unknown-linux-gnu/stage1/lib/rustc/arm-linux-androideabi/lib/libcore-c3ca5d77d81b46c1-0.7-pre.so: error: undefined reference to 'glob'

/opt/ndk_standalone/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: /home/yichoi/rust_work/build/x86_64-unknown-linux-gnu/stage1/lib/rustc/arm-linux-androideabi/lib/libcore-c3ca5d77d81b46c1-0.7-pre.so: error: undefined reference to 'globfre

Since android does not have `glob.h`, `glob_t` definition comes from
https://groups.google.com/forum/?fromgroups=#!topic/android-ndk/vSH6MWPD0Vk

#6100 should be resolved.