Allocate minimal symbols with malloc

Allocate minimal symbols with malloc

Currently, minimal symbols are allocated on the per-BFD obstack.
However, it is also possible for multiple symbol readers to create
minimal symbols for a given objfile.  In this case, the minimal
symbols will be reallocated on the obstack, leading to some waste of
storage.

This is a memory leak, but I think it won't be caught by tools like
valgrind, because valgrind doesn't know about obstacks.

This patch fixes the problem by using malloc to allocate the storage
for minimal symbols.

gdb/ChangeLog
2019-03-15  Tom Tromey  <tom@tromey.com>

	* objfiles.h (struct objfile_per_bfd_storage) <msymbols>: Now a
	unique_xmalloc_ptr.
	(objfile::msymbols_range::begin, objfile::msymbols_range::end):
	Update.
	* minsyms.c (lookup_minimal_symbol_by_pc_section)
	(build_minimal_symbol_hash_tables)
	(minimal_symbol_reader::install): Update.