Have g++ define _FILE_OFFSET_BITS=64 on Solaris

[Rainer Orth]

I recently found two libstdc++ testcases failing on some Solaris hosts
for 32-bit only:

FAIL: 27_io/filesystem/operations/space.cc execution test
FAIL: experimental/filesystem/operations/space.cc execution test

Both file in the same way:

terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'
  what():  filesystem error: cannot get free space: Value too large for defined data type [.]

However, the test PASSes just fine on other systems.

It turns out that the tests FAIL with

statvfs(".", 0xFEFFDB64)                        Err#79 EOVERFLOW

On the failing system, the build filesystem is 3.4 TB, thus the
EOVERFLOW.

It seems g++ on Solaris doesn't fully enable largefile support: it has
-D_LARGEFILE_SOURCE=1 in gcc/config/sol2.h (TARGET_OS_CPP_BUILTINS), but
lacks -D_FILE_OFFSET_BITS=64 which is required to get the
largefile-aware functions (statvfs64 in this case).

The following patch adds that, fixing the two failures.

Bootstrapped without regressions on i386-pc-solaris2.1[01] and
sparc-sun-solaris2.1[01].

Unless someone has an idea why this might cause problems, I'll install
the patch on mainline and backport to the gcc-7 and gcc-8 branches.

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University


2018-06-18  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	* config/sol2.h (TARGET_OS_CPP_BUILTINS): Define
	_FILE_OFFSET_BITS=64 for C++.

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