RFC: target-hook-ize ASM_FILE_START

[Kazu Hirata]

Hi Zack,

> This is decidedly nontrivial and I'd appreciate port maintainers
> checking over their chunk of it.  Problems are especially likely in
> the ARM, PA, and RS/6000 back ends.  There are definitely behavior
> changes in here; I *think* all of them amount to bugfixes.  Please
> note I have not tested this at all.  I expect modifications will be
> necessary, so I'm not going to bother writing a change log for this
> edition.

With this patch, I found no new regressions on h8300 port.  In fact,
the following regressions went away. :-)

FAIL: gcc.dg/c90-intconst-1.c (test for excess errors)
FAIL: gcc.dg/c99-intconst-1.c (test for excess errors)
XPASS: gcc.dg/setjmp-1.c spurious clobbered warning (test for bogus messages, line 16)
FAIL: gcc.dg/setjmp-1.c (test for excess errors)

Kazu Hirata