libsysdecode: Fix some cpp conditionals

While testing the bare values happens to work, the intent is to test
for their definition.

Reviewed by:	emaste
Fixes:		9a16bd169e ("libsysdecode: support errno and syscalls for arm64 Linux")
Fixes:		b69ae1a34c ("libsysdecode: Add preliminary support for decoding Linux syscalls")
Differential Revision:	https://reviews.freebsd.org/D60415

lib/libsysdecode/linux.c

@@ -35,9 +35,9 @@
 
 #ifdef __aarch64__
 #include <arm64/linux/linux.h>
-#elif __i386__
+#elif defined(__i386__)
 #include <i386/linux/linux.h>
-#elif __amd64__
+#elif defined(__amd64__)
 #include <amd64/linux/linux.h>
 #else
 #error "Unsupported Linux arch"

lib/libsysdecode/syscallnames.c

@@ -50,7 +50,7 @@ static
 static
 #ifdef __aarch64__
 #include <arm64/linux/linux_syscalls.c>
-#elif __amd64__
+#elif defined(__amd64__)
 #include <amd64/linux/linux_syscalls.c>
 #else
 #include <i386/linux/linux_syscalls.c>