configure: do not set LLC/OPT/LLVMAS fallback values when FIND_LLVM_PROG fails

When configure fails to find LLC/OPT/LLVMAS within supported version
range, it used to set "llc"/"opt"/"clang" as fallback values. This
behavior is particularly troublesome when the user has llc/opt/clang
with other versions in their PATH and run the testsuite, since hadrian
will incorrectly assume have_llvm=True and pass that to the testsuite
driver, resulting in annoying optllvm test failures (#23186). If
configure determines llc/opt/clang wouldn't work, then we shouldn't
pretend it'll work at all, and the bindist configure will invoke
FIND_LLVM_PROG check again at install time anyway.

(cherry picked from commit 4eb5ad09)

m4/fp_settings.m4

@@ -106,19 +106,10 @@ AC_DEFUN([FP_SETTINGS],
     fi
 
     # LLVM backend tools
-    if test -z "$LlcCmd"; then
-        LlcCmd="llc"
-    fi
     SettingsLlcCommand="$LlcCmd"
 
-    if test -z "$OptCmd"; then
-        OptCmd="opt"
-    fi
     SettingsOptCommand="$OptCmd"
 
-    if test -z "$LlvmAsCmd"; then
-        LlvmAsCmd="clang"
-    fi
     SettingsLlvmAsCommand="$LlvmAsCmd"
 
     # Mac-only tools