LoongArch: Remove useless header compiler.h

mainline inclusion
from mainline-v5.19
commit f62b7626
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

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The content of LoongArch's compiler.h is trivial, with some unused
anywhere, so inline the definitions and remove the header.

Signed-off-by: Jun Yi <yijun@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>