[testsuite][arm] Update warning prune regex

[testsuite][arm] Update warning prune regex

r269586 changed the format of some warning messages.

Each switch in the warning message is now surrounded by single quotes.

This commit updates the regex's in arm.exp dejagnu files to match the
new format and remove the extra 20+ FAILs on excess errors that are
introduced on certain variations (e.g.
arm-eabi-aem/-marm/-march=armv7-a/-mfpu=vfpv3-d16/-mfloat-abi=softfp).

Regtested arm.exp with cross-compiler arm-none-eabi

gcc/testsuite/ChangeLog:

2019-04-12  Matthew Malcomson  <matthew.malcomson@arm.com>

	* g++.target/arm/arm.exp: Change format of default prune regex.
	* gcc.target/arm/arm.exp: Change format of default prune regex.

From-SVN: r270316