gas: improve C_BSTAT and C_STSYM symbols handling on XCOFF

gas: improve C_BSTAT and C_STSYM symbols handling on XCOFF

A C_BSTAT debug symbol specifies the beginning of a static block.
Its n_value is the index of the csect containing static symbols.
A C_STSYM debug symbol represents the stabstring of a statically
allocated symbol. Its n_value is the offset in the csect pointed
by the containing C_BSTAT.

These two special n_value were not correctly handled both when
generating object files with gas or when reading them with objdump.
This patch tries to improve that and, above all, to allow gas-generated
object files with such symbols to be accepted by AIX ld.

bfd/
	* coff-bfd.c (bfd_coff_get_syment): Adjust n_value of symbols
	having fix_value = 1 in order to be an index and not a memory
	offset.
	* coffgen.c (coff_get_symbol_info): Likewize.
	(coff_print_symbol): Likewize.

gas/
	* config/tc-ppc.c (ppc_frob_label): Don't change within if
	already set.
	(ppc_stabx): Remove workaround changing exp.X_add_symbol's
	within.
	* config/tc-ppc.h (struct ppc_tc_sy): Update comments.
	* symbols.c (resolve_symbol_value): Remove symbol update
	when final_val is 0 and it's an AIX debug symbol.
	* testsuite/gas/ppc/aix.exp: Add new tests.
	* testsuite/gas/ppc/xcoff-stsym-32.d: New test.
	* testsuite/gas/ppc/xcoff-stsym-64.d: New test.
	* testsuite/gas/ppc/xcoff-stsym.s: New test.