tried to read symbols from a file, for mapped objfiles. This fixes a
memory leak (actually a constant memory growth) due to reading symbol
files with no debug info and thus generating no psymtabs or symtabs.
Most typically happened with shared libraries.
basis. See comments in objfiles.h and details in ChangeLog. Also remove
redundant definitions of FRAME_CHAIN_VALID from most of the tm-* files and
use a default definition in frame.h.