libstdc++: Fix std::system_category().message(int) on mingw32 target

On the mingw32 target, std::system_category().message(int) uses
FormatMessage api to format error messages. When the error message
contains insert sequences, it is unsafe not to use the
FORMAT_MESSAGE_IGNORE_INSERTS flag, as seen at:
https://devblogs.microsoft.com/oldnewthing/20071128-00/?p=24353

The output of FormatMessage ends with "\r\n" and includes a Full stop
character used by the current thread's UI language. Now, we will remove
"\r\n" and any trailing '.' from the output in any language environment.

In the testsuite for std::system_category().message(int), we first
switch the thread UI language to en-US to meet expectations in any
language environment.

libstdc++-v3/ChangeLog:

	* src/c++11/system_error.cc (system_error_category) [_WIN32]:
	Use FormatMessageA function instead of FormatMessage macro.
	* testsuite/19_diagnostics/error_category/system_category.cc:
	Fix typo in __MINGW32__ macro name.  Adjust behavior on the
	mingw32 target.

(cherry picked from commit 972be84265c2f8a57ca651256c307486482582c6)