[testsuite] Don't overwrite compiler_flags in check_compile

Consider the test-case gcc.c-torture/compile/pr42717.c, which has:
...
/* { dg-xfail-if "ptxas crashes" { nvptx-*-* } { "-O0" } { "" } } */
...

When running make check-gcc, I get:
...
XPASS: gcc.c-torture/compile/pr42717.c   -O0  (test for excess errors)
...
but when forcing to run only that test-case using
RUNTESTFLAGS=compile.exp=pr42717.c I get instead:
...
PASS: gcc.c-torture/compile/pr42717.c   -O0  (test for excess errors)
...

Using RUNTESTFLAGS="-v -v -v" we can see what happens:
...
check_cached_effective_target exceptions_enabled: \
  returning 1 for nvptx-none-run
Limited to targets: *-*-*
Will search for options  "-O0"
Will exclude for options  ""
Compiler flags are: exceptions_enabled9848.cc -fdiagnostics-plain-output \
  --sysroot=/home/vries/nvptx/trunk/install/nvptx-none -S  -isystem \
  /home/vries/nvptx/trunk/build-gcc/nvptx-none/./newlib/targ-include \
  -isystem /home/vries/nvptx/trunk/source-gcc/newlib/libc/include \
  -o exceptions_enabled9848.s
Checking "*-*-*" against "nvptx-unknown-none"
Looking for -O0 to include in the compiler flags
Looking for  to exclude in the compiler flags
This is not a conditional match
PASS: gcc.c-torture/compile/pr42717.c   -O0  (test for excess errors)
...

The effective target exceptions_enabled is tested from gcc-dg-prune, but
the calculation overwrites $compiler_flags, which is subsequently tested for
-O0.

Fix this by saving and restoring $compiler_flags when calling
${tool}_target_compile in check_compile.

Tested on nvptx.

gcc/testsuite/ChangeLog:

2020-10-14  Tom de Vries  <tdevries@suse.de>

	* lib/target-supports.exp (check_compile): Save and restore
	$compiler_flags when calling ${tool}_target_compile.

gcc/testsuite/lib/target-supports.exp

@@ -86,7 +86,10 @@ proc check_compile {basename type contents args} {
     set f [open $src "w"]
     puts $f $contents
     close $f
+    global compiler_flags
+    set save_compiler_flags $compiler_flags
     set lines [${tool}_target_compile $src $output $compile_type "$options"]
+    set compiler_flags $save_compiler_flags 
     file delete $src
 
     set scan_output $output