XCOFF C_STSYM test failure on 32-bit host

This test was failing here and on another similar symbol:
[  4](sec  1)(fl 0x00)(ty   0)(scl 143) (nx 0) 0x05d1745d11745d21 .bs
where correct output is
[  4](sec  1)(fl 0x00)(ty   0)(scl 143) (nx 0) 0x000000000000000a .bs

The problem is caused by a 32-bit host pointer being sign-extended
when stored into a 64-bit bfd_vma, and then that value not being
trimmed back to 32 bits when used.  The following belt-and-braces
patch fixes both the store and subsequent reads.

	* coffcode.h (coff_slurp_symbol_table): Do not sign extend
	when storing a host pointer to syment.n_value.
	* coffgen.c (coff_get_symbol_info): Cast syment.n_value to a
	bfd_hostptr_t before using in arithmetic.
	(coff_print_symbol): Likewise.