drawterm: use a correct atomic exchange for tas

the previous behavior would just keep incrementing the lock,
which is bad; emulate the x86 implementation behavior here by
performing a value exchange instead

we cannot use __atomic_test_and_set (which would eliminate
the value check) since that 1) works on a single byte, which
is okay on little endian systems but bad on big endian systems
and 2) has an undefined value of 'true' (just nonzero)

Fixes https://github.com/void-linux/void-packages/issues/26109

srcpkgs/drawterm/patches/posix_generic.patch

@@ -92,8 +92,8 @@ index 0000000..289f747
 +int
 +tas(int *x)
 +{
-+	/* Use the GCC builtin __sync_fetch_and_add() for optimal code */
-+	int v = __sync_fetch_and_add(x, 1);
++	/* use a gcc __atomic builtin */
++	int v = __atomic_exchange_n(x, 1, __ATOMIC_SEQ_CST);
 +	switch(v) {
 +	case 0:
 +	case 1:

srcpkgs/drawterm/template

@@ -1,7 +1,7 @@
 # Template file for 'drawterm'
 pkgname=drawterm
 version=0.0.20200619
-revision=1
+revision=2
 _hghash=9daaec18b823
 wrksrc=${pkgname}-${_hghash}
 makedepends="libX11-devel libXt-devel"