Fix too-large or negative right shift of negative numbers

As seen in these test failures:

print -1 >> -1
warning: right shift count is negative
$N = 0
(gdb) FAIL: gdb.base/bitshift.exp: lang=c: neg lhs/rhs: print -1 >> -1
print -4 >> -2
warning: right shift count is negative
$N = 0
(gdb) FAIL: gdb.base/bitshift.exp: lang=c: neg lhs/rhs: print -4 >> -2

Fixed by restoring the logic from before the switch to gmp.

Approved-By: Tom Tromey <tom@tromey.com>