I get the exact same problem after upgrading to XCode 3.1. I am building from the command line using gcc-4.2. The problem seems to be caused by having set MallocScribble=1 in the environment. To reproduce run gcc something like
env MallocScribble=1 gcc-4.2 -c helloworld.c
To suppress it do something like:
( unset MallocScribble; gcc-4.2 -c helloworld.c )

This is probably a problem with ‘as’ since the same symptoms happens for gcc-4.2, gcc-4.0, llvm-gcc-4.2

MallocScribble has exposed bugs in the XCode tools before. Some older version of XCode (Perhaps 3.0) had a bug that crashed ‘ld’ when MallocScribble was used.

If MallocScribble is required for this to happen then that may explain why your page is the only google hit for this issue.

“Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!”

He took his vorpal sword in hand:
Long time the manxome foe he sought –
So rested he by the Tumtum tree,
And stood awhile in thought.

And, as in uffish thought he stood,
The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
And burbled as it came!

One, two! One, two! And through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.

“And, has thou slain the Jabberwock?
Come to my arms, my beamish boy!
O frabjous day! Callooh! Callay!’
He chortled in his joy.

`Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

********************
Yes, the above is JABBERWOCKY
by Lewis Carroll (from Through the Looking-Glass and What Alice Found There, 1872)