Avoid compilation errors in MinGW native builds of GDB

Avoid compilation errors in MinGW native builds of GDB

The error is triggered by including python-internal.h, and the
error message is:

     In file included from d:\usr\lib\gcc\mingw32\6.3.0\include\c++\math.h:36:0,
	      from build-gnulib/import/math.h:27,
	      from d:/usr/Python26/include/pyport.h:235,
	      from d:/usr/Python26/include/Python.h:58,
	      from python/python-internal.h:94,
	      from python/py-arch.c:24:
     d:\usr\lib\gcc\mingw32\6.3.0\include\c++\cmath:1157:11: error: '::hypot' has not been declared
        using ::hypot;
	        ^~~~~

This happens because Python headers define 'hypot' to expand to
'_hypot' in the Windows builds.

gdb/ChangeLog:
2018-01-27  Eli Zaretskii  <eliz@gnu.org>

	* python/python-internal.h (_hypot) [__MINGW32__]: Define back to
	'hypoth'.  This avoids a compilation error.