LoongArch: Mark __xchg() and __cmpxchg() as __always_inline

Commit ac7c3e4f ("compiler: enable CONFIG_OPTIMIZE_INLINING
forcibly") allows compiler to uninline functions marked as 'inline'.
In case of __xchg()/__cmpxchg() this would cause to reference
BUILD_BUG(), which is an error case for catching bugs and will not
happen for correct code, if __xchg()/__cmpxchg() is inlined.

This bug can be produced with CONFIG_DEBUG_SECTION_MISMATCH enabled,
and the solution is similar to below commits:
46f16195 ("MIPS: include: Mark __xchg as __always_inline"),
88356d09

 ("MIPS: include: Mark __cmpxchg as __always_inline").

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>