toolchain/nasm: Backport GCC8 compatibility fix from upstream repo

Signed-off-by: Ted Hess <thess@kitschensync.net>

toolchain/nasm/patches/100-backport-upstream-GCC8-compatibility-fixes.patch

@@ -0,0 +1,15 @@
+--- a/include/nasmlib.h
++++ b/include/nasmlib.h
+@@ -188,11 +188,9 @@ int64_t readnum(char *str, bool *error);
+ int64_t readstrnum(char *str, int length, bool *warn);
+ 
+ /*
+- * seg_init: Initialise the segment-number allocator.
+  * seg_alloc: allocate a hitherto unused segment number.
+  */
+-void pure_func seg_init(void);
+-int32_t pure_func seg_alloc(void);
++int32_t seg_alloc(void);
+ 
+ /*
+  * many output formats will be able to make use of this: a standard