commonlib: Substitude macro "__unused" in compiler.h

Since there are many identifiers whose name contain "__unused" in
headers of musl libc, introducing a macro which expands "__unused" to
the source of a util may have disastrous effect during its compiling
under a musl-based platform.

However, it is hard to detect musl at build time as musl is notorious
for having explicitly been refusing to add a macro like "__MUSL__" to
announce its own presence.

Using __always_unused and __maybe_unused for everything may be a good
idea. This is how it works in the Linux kernel, so that would at least
make us match some other standard rather than doing our own thing
(especially since the other compiler.h shorthand macros are also
inspired by Linux).

Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Change-Id: I547ae3371d7568f5aed732ceefe0130a339716a9
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65717
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/riscv/sbi.c

@@ -59,7 +59,7 @@ void handle_sbi(trapframe *tf)
 {
 	uintptr_t ret = 0;
 	uintptr_t arg0 = tf->gpr[10];
-	__unused uintptr_t arg1 = tf->gpr[11];
+	__maybe_unused uintptr_t arg1 = tf->gpr[11];
 	uintptr_t which = tf->gpr[17];
 
 	switch (which) {