[build] Allow assembler section type character to vary by architecture

On some architectures (such as ARM) the "@" character is used as a
comment delimiter.  A section type argument such as "@progbits"
therefore becomes "%progbits".

This is further complicated by the fact that the "%" character has
special meaning for inline assembly when input or output operands are
used, in which cases "@progbits" becomes "%%progbits".

Allow the section type character(s) to be defined via Makefile
variables.

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

src/arch/x86/Makefile

@@ -1,3 +1,8 @@
+# Assembler section type character
+#
+ASM_TCHAR	:= @
+ASM_TCHAR_OPS	:= @
+
 # Include common x86 headers
 #
 INCDIRS		+= arch/x86/include