PR23881, pdp11 binutils fails if too much debug data

PR23881, pdp11 binutils fails if too much debug data

The PR testcase overflows one of the exec header fields, e_syms (the
size of the symbol table), leading to the string table offset being
wrong.  Things go downhill from there.  Fixed by checking for
overflow.  This happens to trigger in the ld testsuite, so xfail that
test.

	PR 23881
bfd/
	* libaout.h (swap_exec_header_out): Return a bool.
	* aoutx.h (swap_exec_header_out): Check for overflow in exec
	header.
	* pdp11.c (swap_exec_header_out): Likewise.
	* i386lynx.c (WRITE_HEADERS): Adjust.
ld/
	* testsuite/ld-scripts/map-address.exp: xfail pdp11.