pkg/compiler: fix handling of defines with a 0 value

While adding new syscall descriptions to OpenBSD I ran into the following problem:

	$ gmake extract SOURCEDIR=/home/src && ./bin/syz-sysgen
	fs.txt.const:15: failed to parse int: strconv.ParseUint: parsing "": invalid syntax
	$ tail -n +15 sys/openbsd/fs.txt.const | head -1
	O_RDONLY =

Note that `O_RDONLY` lacks a value and the same directive is defined as
0 on OpenBSD.

In `ConstFile.Serialize()`, if val is equal to the default value is has
only already been serialized if `max != 0`.

0 files changed, 0 insertions, 0 deletions