Fix backtrace normalization in ice-bug-report-url.rs

This test case currently fails on s390x, and probably other
platforms where the last line of a backtrace does not contain
and " at <source location>" specification.

The problem with the existing normalization lines
// normalize-stderr-test "\s*\d{1,}: .*\n" -> ""
// normalize-stderr-test "\s at .*\n" -> ""
is that \s matches all whitespace, including newlines, so the
first (but not second) of these regexes may merge multiple
lines.  Thus the output differs depending on which of these
matches on the last line of a backtrace.

As the whitespace used in backtraces is just normal space
characters, change both regexes to just match at least one
space character instead:
// normalize-stderr-test " +\d{1,}: .*\n" -> ""
// normalize-stderr-test " + at .*\n" -> ""

tests/rustdoc-ui/ice-bug-report-url.rs

@@ -6,8 +6,8 @@
 // normalize-stderr-test "note: compiler flags.*\n\n" -> ""
 // normalize-stderr-test "note: rustc.*running on.*" -> "note: rustc {version} running on {platform}"
 // normalize-stderr-test "thread.*panicked at .*, compiler.*" -> "thread panicked at 'aborting due to `-Z treat-err-as-bug`'"
-// normalize-stderr-test "\s*\d{1,}: .*\n" -> ""
-// normalize-stderr-test "\s at .*\n" -> ""
+// normalize-stderr-test " +\d{1,}: .*\n" -> ""
+// normalize-stderr-test " + at .*\n" -> ""
 // normalize-stderr-test ".*note: Some details are omitted.*\n" -> ""
 
 fn wrong()

tests/rustdoc-ui/ice-bug-report-url.stderr

@@ -6,6 +6,7 @@ LL | fn wrong()
 
 thread panicked at 'aborting due to `-Z treat-err-as-bug`'
 stack backtrace:
+
 error: the compiler unexpectedly panicked. this is a bug.
 
 note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-rustdoc&template=ice.md