[PowerPC] Detect different long double floating-point formats

[PowerPC] Detect different long double floating-point formats

Current versions of GCC support switching the format used for "long double"
to either IBM double double or IEEE-128.  The resulting binary is marked
via different setting of the Tag_GNU_Power_ABI_FP GNU attribute.

This patch checks this attribute to detect the format of the default
"long double" type and sets GDB's notion of the format accordingly.

The patch also adds support for the "__ibm128" type, which always uses
IBM double double format independent of the format used for "long double".

A new test case verifies that all three types, "long double", "__float128",
and "__ibm128" are correctly detected in all three compiler settings,
the default setting, -mabi=ieeelongdouble, and -mabi=ibmlongdouble.

gdb/ChangeLog:
2017-11-21  Ulrich Weigand  <uweigand@de.ibm.com>

	* ppc-tdep.h (enum powerpc_long_double_abi): New data type.
	(struct gdbarch_tdep): New member long_double_abi.
	* rs6000-tdep.c (rs6000_gdbarch_init): Initialize long_double_abi
	member of tdep struct based on Tag_GNU_Power_ABI_FP attribute.
	* ppc-linux-tdep.c (ppc_linux_init_abi): Install long double data
	format depending on long_double_abi tdep member.
	(ppc_floatformat_for_type): Handle __ibm128 type.

gdb/testsuite/ChangeLog:
2017-11-21  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.arch/ppc-longdouble.exp: New file.
	* gdb.arch/ppc-longdouble.c: Likewise.