configure: Fix detection of SME

Older versions of Clang (Xcode 14, llvm.org Clang 13 and 14)
do support and recognize SME, but when enabled through
".arch_extension sme" it fails to transitively enable support
for Streaming SVE; this was fixed in [1].

This issue results in those versions currently detecting support
for SME, but later failing to build cpu_sme.s with errors like
"error: instruction requires: sve or sme" or "error: instruction
requires: streaming-sve or sve", on the "cntb x0" instruction.

Extend the check for this instruction set extension, to test
with two instructions, both specifically a SME instruction
(smstart) and an instruction that is available in Streaming SVE
mode (cntb).

For the configure check, add an extra parameter to
check_archext_insn for an optional second instruction to check.

It would be tempting to just pass both instructions through
the same parameter, as "smstart; cntb x0". However, Darwin
targets use a different token (%%) for starting a new
instruction on the same line - those targets interpret ";"
as the start of a comment. Due to that, such a check would
entirely ignore the second instruction on Darwin targets.

To avoid dealing with the variability in passing multiple
instructions on one line, just pass the optional second
instruction on a separate line.

[1] ff3f3a54e2

configure

@@ -1207,8 +1207,9 @@ check_archext_insn(){
     log check_archext_insn "$@"
     feature="$1"
     instr="$2"
+    instr2="$3"
     # Check if the assembly is accepted in inline assembly.
-    check_inline_asm ${feature}_inline "\"$instr\""
+    check_inline_asm ${feature}_inline "\"$instr \n\t $instr2\""
     # We don't check if the instruction is supported out of the box by the
     # external assembler (we don't try to set ${feature}_external) as we don't
     # need to use these instructions in non-runtime detected codepaths.
@@ -1230,6 +1231,7 @@ EOF
 $arch_directive
 $arch_extension_directive
 $instr
+$instr2
 EOF
 }
 
@@ -6454,7 +6456,7 @@ if enabled aarch64; then
     enabled i8mm    && check_archext_insn i8mm    'usdot v0.4s, v0.16b, v0.16b'
     enabled sve     && check_archext_insn sve     'whilelt p0.s, x0, x1'
     enabled sve2    && check_archext_insn sve2    'sqrdmulh z0.s, z0.s, z0.s'
-    enabled sme     && check_archext_insn sme     'smstop'
+    enabled sme     && check_archext_insn sme     'smstart' 'cntb x0'
 
     # Disable the main feature (e.g. HAVE_NEON) if neither inline nor external
     # assembly support the feature out of the box. Skip this for the features