Support 'UNSUPPORTED: [...]: exception handling disabled' for libstdc++ testing
Verbatim copy of what was added to 'gcc/testsuite/lib/gcc-dg.exp:gcc-dg-prune'
in Subversion r279246 (Git commit
a9046e9853024206bec092dd63e21e152cb5cbca)
"[MSP430] -Add fno-exceptions multilib".
This greatly improves 'make check-target-libstdc++-v3' results for, for
example, x86_64-pc-linux-gnu with:
RUNTESTFLAGS='--target_board=unix/-fno-exceptions\{,-m32\}'
libstdc++-v3/
* testsuite/lib/prune.exp (libstdc++-dg-prune): Support
'UNSUPPORTED: [...]: exception handling disabled'.