gdb/testsuite: allow gdb.base/overlays.exp to compile for m32r

The gdb.base/overlays.exp test is only currently supported on m32r
baremetal targets, however, when I configure a toolchain for m32r-elf
the test does not compile.

This commit updates the linker script, fixes some TCL errors in the
exp file, and adds some missing includes to the source file so that
the test does compile.

With this test, when run against an m32r-elf toolchain the test mostly
passes, but there are a couple of failures, these are GDB issues and
will be addressed in a later commit.

gdb/testsuite/ChangeLog:

	* gdb.base/m32r.ld: Remove SEARCH_DIR line.  Add MEMORY regions,
	make use of regions throughout.
	* gdb.base/overlays.exp: Enclose string with variableds in "..",
	not {...}.
	* gdb.base/ovlymgr.c: Add 'string.h' and 'stdlib.h' includes.