[gdb/testsuite] Check avx support in gdb.arch/amd64-disp-step-avx.exp

On a machine on Open Build Service I'm running into a SIGILL for test-case
gdb.arch/amd64-disp-step-avx.exp:
...
Program received signal SIGILL, Illegal instruction.^M
test_rip_vex2 () at gdb.arch/amd64-disp-step-avx.S:40^M
40              vmovsd ro_var(%rip),%xmm0^M
(gdb) FAIL: gdb.arch/amd64-disp-step-avx.exp: vex2: \
  continue to test_rip_vex2_end
...
The SIGILL happens when trying to execute the first avx instruction in the
executable.

I can't directly access the machine, but looking at the log for test-case
gdb.arch/i386-avx.exp, it seems that there's no avx support:
...
Breakpoint 1, main (argc=1, argv=0x7fffffffd6b8) at gdb.arch/i386-avx.c:68^M
68        if (have_avx ())^M
(gdb) print have_avx ()^M
$1 = 0^M
...

Fix this by:
- adding a gdb_caching_proc have_avx, similar to have_mpx, using the have_avx
  function from gdb.arch/i386-avx.c
- using proc have_avx in both gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp
  and gdb/testsuite/gdb.arch/i386-avx.exp.

Tested on my x86_64-linux laptop with avx support, where both test-cases pass.

gdb/testsuite/ChangeLog:

2021-09-04  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26950
	* gdb/testsuite/gdb.arch/i386-avx.c (main): Remove call to have_avx.
	(have_avx): Move ...
	* gdb/testsuite/lib/gdb.exp (have_avx): ... here.  New proc.
	* gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp: Use have_avx.
	* gdb/testsuite/gdb.arch/i386-avx.exp: Same.

gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp

@@ -23,6 +23,11 @@ if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
     return
 }
 
+if { ![have_avx] } {
+    verbose "Skipping x86_64 displaced stepping tests."
+    return
+}
+
 standard_testfile .S
 
 set options [list debug \

gdb/testsuite/gdb.arch/i386-avx.c

@@ -48,24 +48,8 @@ v8sf_t data[] =
   };
 
 
-int
-have_avx (void)
-{
-  unsigned int eax, ebx, ecx, edx;
-
-  if (!x86_cpuid (1, &eax, &ebx, &ecx, &edx))
-    return 0;
-
-  if ((ecx & (bit_AVX | bit_OSXSAVE)) == (bit_AVX | bit_OSXSAVE))
-    return 1;
-  else
-    return 0;
-}
-
 int
 main (int argc, char **argv)
-{
-  if (have_avx ())
 {
   asm ("vmovaps 0(%0), %%ymm0\n\t"
        "vmovaps 32(%0), %%ymm1\n\t"
@@ -122,7 +106,6 @@ main (int argc, char **argv)
 #endif
 
   puts ("Bye!"); /* second breakpoint here */
-    }
 
   return 0;
 }

gdb/testsuite/gdb.arch/i386-avx.exp

@@ -24,6 +24,11 @@ if { ![istarget i?86-*-*] && ![istarget x86_64-*-* ] } {
     return
 }
 
+if { ![have_avx] } {
+    verbose "Skipping x86 AVX tests."
+    return
+}
+
 standard_testfile .c
 
 if [get_compiler_info] {
@@ -47,23 +52,6 @@ if ![runto_main] then {
     return 0
 }
 
-send_gdb "print have_avx ()\r"
-gdb_expect {
-    -re ".. = 1\r\n$gdb_prompt " {
-        pass "check whether processor supports AVX"
-    }
-    -re ".. = 0\r\n$gdb_prompt " {
-        verbose "processor does not support AVX; skipping AVX tests"
-        return
-    }
-    -re ".*$gdb_prompt $" {
-        fail "check whether processor supports AVX"
-    }
-    timeout {
-        fail "check whether processor supports AVX (timeout)"
-    }
-}
-
 gdb_test "break [gdb_get_line_number "first breakpoint here"]" \
          "Breakpoint .* at .*i386-avx.c.*" \
          "set first breakpoint in main"

gdb/testsuite/lib/gdb.exp

@@ -8123,5 +8123,49 @@ gdb_caching_proc have_mpx {
     return $status
 }
 
+# Return 1 if target supports avx, otherwise return 0.
+gdb_caching_proc have_avx {
+    global srcdir
+
+    set me "have_avx"
+    if { ![istarget "i?86-*-*"] && ![istarget "x86_64-*-*"] } {
+        verbose "$me: target does not support avx, returning 0" 2
+        return 0
+    }
+
+    # Compile a test program.
+    set src {
+       #include "nat/x86-cpuid.h"
+
+	int main() {
+	  unsigned int eax, ebx, ecx, edx;
+
+	if (!x86_cpuid (1, &eax, &ebx, &ecx, &edx))
+	  return 0;
+
+	if ((ecx & (bit_AVX | bit_OSXSAVE)) == (bit_AVX | bit_OSXSAVE))
+	  return 1;
+	else
+	  return 0;
+	}
+    }
+    set compile_flags "incdir=${srcdir}/.."
+    if {![gdb_simple_compile $me $src executable $compile_flags]} {
+        return 0
+    }
+
+    set result [remote_exec target $obj]
+    set status [lindex $result 0]
+    set output [lindex $result 1]
+    if { $output != "" } {
+	set status 0
+    }
+
+    remote_file build delete $obj
+
+    verbose "$me: returning $status" 2
+    return $status
+}
+
 # Always load compatibility stuff.
 load_lib future.exp