[build] Fix use of inline assembly on GCC 4.8 ARM64 builds

The inline assembly used in include/errno.h to generate the einfo
blocks requires the ability to generate an immediate constant with no
immediate-value prefix (such as the dollar sign for x86 assembly).

We currently achieve this via the undocumented "%c0" form of operand.
This causes an "invalid operand prefix" error on GCC 4.8 for ARM64
builds.

Fix by switching to the equally undocumented "%a0" form of operand,
which appears to work correctly on all tested versions of GCC.

Reported-by: Benjamin S. Allen <bsallen@alcf.anl.gov>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/errno.h

@@ -262,10 +262,10 @@ static inline void eplatform_discard ( int dummy __unused, ... ) {}
 		  ".align 8\n\t"					\
 		  "\n1:\n\t"						\
 		  ".long ( 4f - 1b )\n\t"				\
-		  ".long %c0\n\t"					\
+		  ".long %a0\n\t"					\
 		  ".long ( 2f - 1b )\n\t"				\
 		  ".long ( 3f - 1b )\n\t"				\
-		  ".long %c1\n\t"					\
+		  ".long %a1\n\t"					\
 		  "\n2:\t.asciz \"" __einfo_desc ( einfo ) "\"\n\t"	\
 		  "\n3:\t.asciz \"" __FILE__ "\"\n\t"			\
 		  ".align 8\n\t"					\