Demangle minsyms in parallel

This patch introduces a simple parallel for_each and changes the
minimal symbol reader to use it when computing the demangled name for
a minimal symbol.  This yields a speedup when reading minimal symbols.

gdb/ChangeLog
2019-09-30  Tom Tromey  <tom@tromey.com>

	* minsyms.c (minimal_symbol_reader::install): Use
	parallel_for_each.
	* gdbsupport/parallel-for.h: New file.
	* gdbsupport/parallel-for.c: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add gdbsupport/parallel-for.h.
	(COMMON_SFILES): Add gdbsupport/parallel-for.c.
5 files changed