libc: Add support for static_assert

The C11 standard requires this. From 7.2 "Diagnostics <assert.h>"
paragraph 1:
> The header <assert.h> defines the assert and static_assert macros...

paragraph 3:
> The macro
>     static_assert
> expands to _Static_assert.

Since static_assert is a keyword in C++11, don't define it if C++.

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>