[vta,vta4.3] handle negative bitpos expanding debug stmts

[Alexandre Oliva]

crtstuff computes __CTOR_END__ - 1, and this ICEd while testing
-fcompare-debug.  It's normally compiled with -g0, but I introduced
-gtoggle for -fcompare-debug, so it ended up compiled for comparison
with -g2, and the assignment p = __CTOR_END__ - 1; generated a debug
stmt with the expression &__CTOR_END__[-1], which exposed the failure
of expand_debug_expr() to handle negative bitpos.  Fixed as follows.

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-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}
FSFLA Board Member       ?S? Libre! => http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}