unified/x86: fix IAMCU build

Unified kernel does not provide the _thread_arg_t type, but instead uses
void * directly for its thread entry parameters. _thread_entry_t is
typedefed from void * anyway, and only obfuscates the type. So, define
_thread_entry_t to be a function pointer to a function with three void *
parameters, and when the unified kernel becomes the only kernel, all the
_thread_arg_t types will go away.

With this change, IAMCU runs all the tests sysV x86 is able to run as a
unified kernel.

Change-Id: I53c8754629a5a0a114a16a775ff1efc1884496ff
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>