sysdeps: arm: Fix preconfigure script for ARMv8/v9 targets [BZ #29698]

The ARM preconfigure script tries to detect the capabilities of the
target platform by checking the compiler's predefined architecture
macros. However, if the compiler is tuning for AArch32 on ARMv8/v9 this
step fails:

checking for sysdeps preconfigure fragments... aarch64 alpha arc arm
WARNING: arm/preconfigure: Did not find ARM architecture type; using default

This is because preconfigure.ac doesn't escape the square brackets in
the glob for matching compilers targeting ARMv8. Adding another pair of
brackets to escape the first pair fixes this:

checking for sysdeps preconfigure fragments... aarch64 alpha arc arm
 Found compiler is configured for something newer than v7 - using v7

Signed-off-by: Felix Riemann <felix.riemann@sma.de>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>