stubs: export symbols properly

These two symbols were not correctly scoped, placing them inside of the swift
namespace.  This would change the linkage names and fail to link as a result.
Scope one as the header properly places it in an extern "C" block.  The gyb file
is unable to find the desired runtime header, so explicitly mark the exported
symbol as being exposed with C linkage.

Author: compnerd

stdlib/public/stubs/LibcShims.cpp

@@ -93,7 +93,7 @@ size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
 }
 #elif defined(_MSC_VER)
 #include <malloc.h>
-size_t _swift_stdlib_malloc_size(const void *ptr) {
+size_t swift::_swift_stdlib_malloc_size(const void *ptr) {
   return _msize(const_cast<void *>(ptr));
 }
 #elif defined(__FreeBSD__)

stdlib/public/stubs/UnicodeExtendedGraphemeClusters.cpp.gyb

@@ -131,6 +131,7 @@ const uint16_t _swift_stdlib_ExtendedGraphemeClusterNoBoundaryRulesMatrixImpl[]
 % end
 };
 
+extern "C"
 SWIFT_RUNTIME_STDLIB_INTERFACE
 const uint16_t *_swift_stdlib_ExtendedGraphemeClusterNoBoundaryRulesMatrix =
     _swift_stdlib_ExtendedGraphemeClusterNoBoundaryRulesMatrixImpl;