Test on solib load and unload

This patch is to add a test case to on the performance of GDB handling
load and unload of shared library.

In V4:

 - Handle malloc and dlopen failure,
 - Document test parameters.

In V3, there are some changes,

 - Adapt to perf test framework changes.
 - Measure load and unload separately.

In V2, there are some changes,

 - A new proc gdb_produce_source to produce source files.  I tried to
   move all source file generation code out of solib.exp, but
   compilation step still needs to know the generated file names.  I
   have to hard-code the file names in compilation step, which is not
   good to me, so I give up on this moving.
 - SOLIB_NUMBER -> SOLIB_COUNT
 - New variable SOLIB_DLCLOSE_REVERSED_ORDER to control the order of
   iterating a list of shared libs to dlclose them.
 - New variable GDB_PERFORMANCE to enable these perf test cases.
 - Remove dlsym call in solib.c.
 - Update solib.py for the updated framework.

gdb/testsuite/

	* lib/gdb.exp (gdb_produce_source): New procedure.
	* gdb.perf/solib.c: New.
	* gdb.perf/solib.exp: New.
	* gdb.perf/solib.py: New.

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