s390/lowcore: Use inline qualifier for get_lowcore() inline assembly

Use asm_inline to let the compiler know that the get_lowcore() inline
assembly has the smallest possible size. The ALTERNATIVE construct is used
to generate a single instruction, however the macro expands to multiple
lines. GCC uses the number of lines of an inline assembly to count the
number of instructions within an inline assembly, which then has an effect
on inlining decisions.

In order to avoid incorrect assumptions use asm_inline. The result is that
more functions are inlined, which results in a small growth of the kernel
image:

add/remove: 59/480 grow/shrink: 854/647 up/down: 168780/-162394 (6386)

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>