[PATCH v2] configure: define TARGET_LIBC_GNUSTACK on musl

musl only uses PT_GNU_STACK to set default thread stack size and has no
executable stack support[0], so there is no reason not to emit the
.note.GNU-stack section on musl builds.

[0]: https://lore.kernel.org/all/20190423192534.GN23599@brightrain.aerifal.cx/T/#u

gcc

	* configure: Regenerate.
	* configure.ac: Define TARGET_LIBC_GNUSTACK on musl.