LoongArch: Replace __ASSEMBLY__ with __ASSEMBLER__ in headers

While the GCC and Clang compilers already define __ASSEMBLER__
automatically when compiling assembler code, __ASSEMBLY__ is a macro
that only gets defined by the Makefiles in the kernel. This is bad
since macros starting with two underscores are names that are reserved
by the C language. It can also be very confusing for the developers
when switching between userspace and kernelspace coding, or when
dealing with uapi headers that rather should use __ASSEMBLER__ instead.
So let's now standardize on the __ASSEMBLER__ macro that is provided
by the compilers.

This is almost a completely mechanical patch (done with a simple
"sed -i" statement), with one comment tweaked manually in the
arch/loongarch/include/asm/cpu.h file (it was missing the trailing
underscores).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>