[gdb/build,c++17] Fix use of deprecated std::uncaught_exception

[gdb/build,c++17] Fix use of deprecated std::uncaught_exception

When compiling gdb with -std=gnu++17, we run into:
...
../../gdb/inferior.h: In member function ‘void \
  infcall_suspend_state_deleter::operator()(infcall_suspend_state*) const’:
../../gdb/inferior.h:83:12: error: ‘bool std::uncaught_exception()’ is \
  deprecated [-Werror=deprecated-declarations]
   83 |  if (!std::uncaught_exception ())
...

Fix this by rewriting using std::uncaught_exceptions.

Tested on x86_64-linux with gcc 9.3.1 and -std=gnu17/gnu++17.

Tested with test-case from RFC patch
https://sourceware.org/pipermail/gdb-patches/2020-June/169970.html.

gdb/ChangeLog:

2020-07-05  Tom de Vries  <tdevries@suse.de>

	PR build/26187
	* inferior.h (struct infcall_suspend_state_deleter): If available, use
	std::uncaught_exceptions instead of deprecated
	std::uncaught_exception.