libc: fix sparse warning for __assert()

Sparse warns this:

lib/libc/assert.c:29:6: error: symbol '__assert' redeclared with different type (originally declared at include/lib/libc/assert.h:36) - different modifiers

Add __dead2 to match the header declaration and C definition.

I also changed '__dead2 void' to 'void __dead2' for the consistency
with other parts.

Change-Id: Iefa4f0e787c24fa7e7e499d2e7baf54d4deb49ef
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>