configure: Add braces to clarify intent of $emu[[:space:]]

In shell script syntax, $var[something] is not special for variable
expansion: $var is expanded.  However, as it can look as if it were
intended to be an array element access (the correct syntax for which
is ${var[something]}), shellcheck recommends using explicit braces
around ${var} to clarify the intended expansion.

This fixes the warning:

In ./configure line 2346:
        if "$target_ld" -verbose 2>&1 | grep -q "^[[:space:]]*$emu[[:space:]]*$"; then
                                                              ^-- SC1087: Use braces when expanding arrays, e.g. ${array[idx]} (or ${var}[.. to quiet).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20220720152631.450903-3-peter.maydell@linaro.org