alpha: remove undef inline in compiler.h

since 889b3c12, CONFIG_OPTIMIZE_INLINING
is removed entirely and inline is always defined to `inline __gnu_inline
__inline_maybe_unused notrace` in compiler_types.h

Besides, undef inline here also means it never use
__attribute__((__gnu_inline__)), so `extern inline` function can never
be defined header files, otherwise multiple definition errors will
happen, e.g. if multiple translation units use alpha/include/asm/pal.h
will report multiple definitions, because there are many extern inline
function definitions in this header.

``` c
extern inline TYPE NAME(void)					\
{								\
	register TYPE __r0 __asm__("$0");			\
	__asm__ __volatile__(					\
...
```

Ofc, it is also ok to remove `extern` in `extern inline` here, then all
of iso c99 and gnuc99/89 are ok, but there are also other alpha headers
have such function definitions.

Signed-off-by: chenli <chenli@uniontech.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>