configure.ac: strip out the llvm-config -march/mtune flags

Otherwise drivers such as SWR that depend on providing their own values
will fail to build.

v2: Add -mcpu for good measure (Chuck)

Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Chuck Atkins <chuck.atkins@kitware.com>
Tested-by: Chuck Atkins <chuck.atkins@kitware.com>

configure.ac

@@ -2102,6 +2102,9 @@ AC_ARG_WITH([llvm-prefix],
 strip_unwanted_llvm_flags() {
     # Use \> (marks the end of the word)
     echo `$1` | sed \
+	-e 's/-march=\S*//g' \
+	-e 's/-mtune=\S*//g' \
+	-e 's/-mcpu=\S*//g' \
 	-e 's/-DNDEBUG\>//g' \
 	-e 's/-D_GNU_SOURCE\>//g' \
 	-e 's/-pedantic\>//g' \