targets except some specific ones that are known to not support signals.
Also clean up formatting.
* gdb.base/Makefile.in (EXECUTABLES): Remove sigall and signals.
(signals, sigall): Remove rules to build.
* gdb.base/{sigall.c, sigall.exp, signals.c, signals.exp}:
Moved to new gdb.signals directory.
* gdb.signals: New test directory.
* gdb.signals/{Makefile.in, configure.in, sigall.c, sigall.exp,
signals.c signals.exp}: New or moved files.
