enhance(main/libuv): Check for TERMUX_EXEC__PROC_SELF_EXE

[fornwall]

This fixes running e.g. neovim under termux-exec with updated targetSdk.

Brief summary: When running with targetSdk to 28 or higher, it's not allowed to execute downloaded files (files not part of the APK in general) directly.

The termux-exec workaround using LD_PRELOAD that is currently used in the Google Play build instead executes e.g. /system/bin/linker64 $PREFIX/bin/nvim instead of $PREFIX/bin/nvim directly. One consequence of that is that reading /proc/self/exe to get the path to the currently running executable does not work, so patches like these (that should be a no-op when not running under bumped targetSdk and termux-exec) are necessary.

[fornwall]

I've raised the question in the termux/dev chat, if we want to start merging these kind of patches to prepare for this repository serving both targetSdk<29 and targetSdk>=29 builds.

[Grimler91]

Before merging these type of PRs we should land termux/termux-exec#24 and #18872. Will look at it again