Utilize Android NDK installations

NDK comes with a handful of compilers and this patch modifies gcc-explorer to utilize them.

Standalone invocations of toolchains inside NDK requires --sysroot
option to be set to a correct 'system/platform' directory inside NDK
however this isn't necessary for trivial compilations.

Tested with NDK r9c.

[1]:
https://android.googlesource.com/platform/ndk.git/+/master/docs/text/STANDALONE-TOOLCHAIN.text