Support 128-bit IEEE floating-point types on Intel and Power

Support 128-bit IEEE floating-point types on Intel and Power

Now that all the prerequisites are in place, this commit finally adds support
for handling the __float128 type on Intel and Power, by providing appropriate
platform-specific versions of the floatformat_for_type callback.

Since at this point we do not yet have any indication in the debug info to
distinguish different floating-point formats of the same length, we simply
use the type name as hint.  Types named "__float128" get the IEEE format.
In addition to handling "__float128" itself, we also recognize "_Float128"
and (on Power) "_Float64x", as well as the complex versions of those.
(As pointed out by Joseph Myers, starting with GCC 7, __float128 is just
a typedef for _Float128 -- but it's good to handle this anyway.)

A new test case does some simple verification that the format is decoded
correctly, using both __float128 and "long double" to make sure using both
in the same file still works.  Another new test verifies handling of the
_FloatN and _FloatNx types supported by GCC 7, as well as the complex
versions of those types.

Note that this still only supports basic format decoding and encoding.
We do not yet support the GNU extension 'g' suffix for __float128 constants.
In addition, since all *arithmetic* on floating-point values is still
performed in native host "long double" arithmetic, if that format is not
able to encode all target __float128 values, we may get incorrect results.
(To fix this would require implementing fully synthetic target floating-
point arithmetic along the lines of GCC's real.c, presumably using MPFR.)

gdb/ChangeLog:

	* i386-tdep.c (i386_floatformat_for_type): New function.
	(i386_gdbarch_init): Install it.
	* ppc-linux-tdep.c (ppc_floatformat_for_type): New function.
	(ppc_linux_init_abi): Install it.

gdb/testsuite/ChangeLog:

	* gdb.base/float128.c: New file.
	* gdb.base/float128.exp: Likewise.
	* gdb.base/floatn.c: Likewise.
	* gdb.base/floatn.exp: Likewise.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>