feat(glibc_2_32): add polyfill for __libc_single_threaded, constant 0

[tuxxi]

C.f. #12

https://www.gnu.org/software/libc/manual/html_node/Single_002dThreaded.html

Variable: char __libc_single_threaded
This variable is non-zero if the current process is definitely single-threaded. If it is zero, the process may be multi-threaded, or the GNU C Library cannot determine at this point of the program execution whether the process is single-threaded or not.

For now this is a constant zero.

A future enhancement to do this better, would be to detect if any functions from pthread are called, or if libpthread itself is linked dynamically to the binary -- then set this value to nonzero if so.

[IoIxD]

Tested this with my own program, can confirm it works (given the program isn't multi threaded lol).