Redefine gdb_static_assert as static_assert

Redefine gdb_static_assert as static_assert

Since we use C++11, we can use static_assert instead doing the trick
that makes a negative-sized array if the expression is false.
static_assert is built in the language and gives clearer error messages.

To avoid modifying the usages of gdb_static_assert, redefine
gdb_static_assert in terms of static_assert, passing an empty message.
If we want to add an assert with a message, it's always possible to use
static_assert directly.

gdb/ChangeLog:

	* common/gdb_assert.h (gdb_static_assert): Redefine using
	static_assert.