directive: Make cpu directive case insensitive back again

During code suffle we occasionally made cpu directive to
take letter case into account despite the documentation.

https://bugzilla.nasm.us/show_bug.cgi?id=3392491

Reported-by: Rebecca Cran <rebecca@bluestop.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

asm/directiv.c

@@ -102,7 +102,7 @@ static iflag_t get_cpu(const char *value)
     iflag_clear_all(&r);
 
     for (cpu = cpunames; cpu->name; cpu++) {
-        if (!strcmp(value, cpu->name))
+        if (!nasm_stricmp(value, cpu->name))
             break;
     }