[build] Remove redundant regparm function attribute for x86_64

The regparm function attribute is meaningful only for i386, not for
x86_64, and is reported as a build error by GCC 16.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86_64/include/bits/compiler.h

@@ -13,7 +13,7 @@ FILE_SECBOOT ( PERMITTED );
 #define ASM_NO_PREFIX "c"
 
 /** Declare a function with standard calling conventions */
-#define __asmcall __attribute__ (( regparm(0) ))
+#define __asmcall
 
 /** Declare a function with libgcc implicit linkage */
 #define __libgcc