Remove all shared library dependencies to support running in both Bionic and glibc

Currently using termux-exec as LD_PRELOAD causes things to fail if loading a glibc-based executable with a bionic-based termux-exec preload or viceversa a bionic-based executable with a glibc-based termux-exec preload.

This makes it impossible, for instance, to run, from a glibc-based program, a script using "#!/usr/bin/env node" where node uses bionic, because to run it LD_PRELOAD has to be set to a glibc-based termux-exec, but that will cause a failure to run node.

The solution is to make termux-exec loadable in both glibc and bionic based programs, which should be achievable by not depending on any libc and instead doing raw Linux system calls in termux-exec, and exporting both Bionic-flavored and glibc-flavored exec* functions if they differ.

Start by locating the termux-exec implementation and its build configuration, then inspect which shared-library and libc dependencies it currently uses. Verify how the exported exec* functions differ between Bionic and glibc. Done means termux-exec can be preloaded by both Bionic- and glibc-based programs without loading failures.