add freebsd support

[hoanga]

the following changeset allows go-llvm (as well as tinygo) to compile on freebsd (tested on freebsd 14.3). without the changes applied the following error occurs when trying to compile on freebsd

$ uname -a
FreeBSD quincy 14.3-RELEASE FreeBSD 14.3-RELEASE releng/14.3-n271432-8c9ce319fef7 GENERIC amd64

$ go build ./...
# tinygo.org/x/go-llvm
./bitwriter.go:16:10: fatal error: 'llvm-c/BitWriter.h' file not found
   16 | #include "llvm-c/BitWriter.h"
      |          ^~~~~