x86/alternative: Refactor INT3 call emulation selftest

x86/alternative: Refactor INT3 call emulation selftest

The INT3 call emulation selftest is a bit fragile as it relies on the
compiler not inserting any extra instructions before the
int3_selftest_ip() definition.

Also, the int3_selftest_ip() symbol overlaps with the int3_selftest
symbol(), which can confuse objtool.

Fix those issues by slightly reworking the functionality and moving
int3_selftest_ip() to a separate asm function.  While at it, improve the
naming.

Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>