Add libmsvcrt, libmsvcrt-os and libucrtbase to the list of libraries for which the automatic generation of exported variables should be disabled.

In the latest git version of mingw-w64, one can set the default
libmsvcrt.a to actually be the import library of another CRT version,
to facilitate switching to ucrtbase.dll. In these versions, the
traditional libmsvcrt.a (for linking to msvcrt.dll) is also provided
with the name libmsvcrt-os.a.

The import libraries for the CRT also contain a number of compatibility
wrapper functions. The ones in libmsvcrt.a have so far been excluded
automatically since they in most cases come with a corresponding
__imp_foo function. None of these make sense to export automatically
from a DLL though, so add these libraries to the exclude list.

libucrtbase.a contains a number of compatibility wrapper functions
that don't have a corresponding __imp_foo symbol, which cause
these symbols to be exported from DLLs.

This can be worked around on the mingw-w64 side by adding extra
__imp_ symbols, but since they aren't strictly necessary for linking
to succeed, it's probably better long-term to add these libraries to
the exclusion list.

ld	* pe-dll.c (autofilter_liblist): Add libmsvcrt, libmsvcrt-os and
	libucrtbase.

ld/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-23  Martin Storsjo  <martin@martin.st>
+
+	* pe-dll.c (autofilter_liblist): Add libmsvcrt, libmsvcrt-os and
+	libucrtbase.
+
 2017-11-23  Jan Beulich  <jbeulich@suse.com>
 
 	PR gas/22441

ld/pe-dll.c

@@ -344,6 +344,9 @@ static const autofilter_entry_type autofilter_liblist[] =
   { STRING_COMMA_LEN ("libsupc++") },
   { STRING_COMMA_LEN ("libobjc") },
   { STRING_COMMA_LEN ("libgcj") },
+  { STRING_COMMA_LEN ("libmsvcrt") },
+  { STRING_COMMA_LEN ("libmsvcrt-os") },
+  { STRING_COMMA_LEN ("libucrtbase") },
   { NULL, 0 }
 };