[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/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