builtin: reduce a bit the generated #if defined checks for small programs

[spytheman]

For a hello world program, the C code reduction is a few hundred bytes:

#0 13:27:38 ^ master ~/code/v>ls -lart old.c new.c new2.c new3.c new4.c new5.c new6.c
-rw-rw-r-- 1 delian delian 45569 Jan 16 12:59 old.c
-rw-rw-r-- 1 delian delian 45207 Jan 16 12:59 new.c
-rw-rw-r-- 1 delian delian 45367 Jan 16 13:07 new2.c
-rw-rw-r-- 1 delian delian 45259 Jan 16 13:08 new3.c
-rw-rw-r-- 1 delian delian 45102 Jan 16 13:09 new4.c
-rw-rw-r-- 1 delian delian 45102 Jan 16 13:19 new5.c
-rw-rw-r-- 1 delian delian 45102 Jan 16 13:25 new6.c

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