Because start.s includes disklabel.h, which includes sys/types.h,
which includes machine/types.h, which includes machine/int_types.h,
the assembler is asked to handle typedef statements, and can't.

cc -x assembler-with-cpp  -D_STANDALONE -DLIBSA_NO_FD_CHECKING -DLIBSA_NO_RAW_ACCESS  -DLIBSA_NO_TWIDDLE -DLIBSA_SINGLE_DEVICE=rom  -DLIBSA_NO_COMPAT_UFS  -DLIBSA_NO_FS_SYMLINK -DLIBSA_NO_FS_CLOSE  -DLIBSA_NO_FS_WRITE -DLIBSA_NO_FS_SEEK -I. -I/usr/src/sys/arch/vax/boot/xxboot/../../../../ -I/usr/src/sys/arch/vax/boot/xxboot/../../ -I/usr/src/sys/arch/vax/boot/xxboot/../common -I/usr/src/sys/arch/vax/boot/xxboot/../../include -DRELOC=0x2f0000 -E /usr/src/sys/arch/vax/boot/xxboot/start.s | as -o start.o
In file included from /usr/src/sys/arch/vax/boot/xxboot/start.s:44:
/usr/src/sys/arch/vax/boot/xxboot/../../include/../include/asm.h:57: warning: `_C_LABEL' redefined
/usr/src/sys/arch/vax/boot/xxboot/../../../../sys/cdefs_aout.h:11: warning: this is the location of the previous definition
machine/int_types.h: Assembler messages:
machine/int_types.h:47: Warning: Ignoring statement due to "Unknown operator"
machine/int_types.h:48: Warning: Ignoring statement due to "Unknown operator"

...and so on.

There's also that redefinition of _C_LABEL(), which needs fixing...

-tih
-- 
The basic difference is this: hackers build things, crackers break them.