build: fix broken configure

The configure generated by Ubuntu22's upstream autoconf (2.71) is broken.
DEFAULT_VARCH was expanded incorrectly, causing the generated line to be truncated from the comma.
Since this macro is a constant and doesn't depend on environment variables, it doesn't require shell expansion.

Author: Phantom1003

riscv/riscv.ac

@@ -24,7 +24,7 @@ AC_ARG_WITH(varch,
 	[AS_HELP_STRING([--with-varch=vlen:128,elen:64],
 		[Sets the default vector config])],
   AC_DEFINE_UNQUOTED([DEFAULT_VARCH], "$withval", [Default value for --varch switch]),
-  AC_DEFINE_UNQUOTED([DEFAULT_VARCH], ["vlen:128,elen:64"], [Default value for --varch switch]))
+  AC_DEFINE([DEFAULT_VARCH], ["vlen:128,elen:64"], [Default value for --varch switch]))
 
 AC_ARG_WITH(target,
 	[AS_HELP_STRING([--with-target=riscv64-unknown-elf],