[build] Allow for per-architecture unprefixed constant operand modifier

Over the years, the undocumented operand modifier used to produce the
unprefixed constant values in __einfo_error() has varied from "%c0" to
"%a0" in commit 1a77466 ("[build] Fix use of inline assembly on GCC
4.8 ARM64 builds") and back to "%c0" in commit 3fb3ffc ("[build] Fix
use of inline assembly on GCC 8 ARM64 builds"), according to the
evolving demands of the toolchain.

LoongArch64 suffers from a similar issue: GCC 13 will allow either,
but the currently released GCC 12 allows only the "%a0" form.

Introduce a macro ASM_NO_PREFIX, defined in bits/compiler.h, to
abstract away this difference and allow different architectures to use
different operand modifiers.

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

src/arch/arm32/include/bits/compiler.h

@@ -8,6 +8,9 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #ifndef ASSEMBLY
 
+/** Unprefixed constant operand modifier */
+#define ASM_NO_PREFIX "c"
+
 #define __asmcall
 #define __libgcc
 

src/arch/arm64/include/bits/compiler.h

@@ -8,6 +8,9 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #ifndef ASSEMBLY
 
+/** Unprefixed constant operand modifier */
+#define ASM_NO_PREFIX "c"
+
 #define __asmcall
 #define __libgcc
 

src/arch/i386/include/bits/compiler.h

@@ -8,6 +8,9 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #ifndef ASSEMBLY
 
+/** Unprefixed constant operand modifier */
+#define ASM_NO_PREFIX "c"
+
 /** Declare a function with standard calling conventions */
 #define __asmcall __attribute__ (( cdecl, regparm(0) ))
 

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

@@ -8,6 +8,9 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #ifndef ASSEMBLY
 
+/** Unprefixed constant operand modifier */
+#define ASM_NO_PREFIX "c"
+
 /** Declare a function with standard calling conventions */
 #define __asmcall __attribute__ (( regparm(0) ))
 

src/include/errno.h

@@ -262,10 +262,10 @@ static inline void eplatform_discard ( int dummy __unused, ... ) {}
 		  ".balign 8\n\t"					\
 		  "\n1:\n\t"						\
 		  ".long ( 4f - 1b )\n\t"				\
-		  ".long %c0\n\t"					\
+		  ".long %" ASM_NO_PREFIX "0\n\t"			\
 		  ".long ( 2f - 1b )\n\t"				\
 		  ".long ( 3f - 1b )\n\t"				\
-		  ".long %c1\n\t"					\
+		  ".long %" ASM_NO_PREFIX "1\n\t"			\
 		  "\n2:\t.asciz \"" __einfo_desc ( einfo ) "\"\n\t"	\
 		  "\n3:\t.asciz \"" __FILE__ "\"\n\t"			\
 		  ".balign 8\n\t"					\