gdb: Improve formatting of 'show architecture' messages

This commit changes the output of 'show architecture'.  Here is a
session before this commit:

  (gdb) show architecture
  The target architecture is set automatically (currently i386)
  (gdb) set architecture mips
  The target architecture is assumed to be mips
  (gdb) show architecture
  The target architecture is assumed to be mips
  (gdb)

After this commit the session now looks like this:

  (gdb) show architecture
  The target architecture is set to "auto" (currently "i386").
  (gdb) set architecture mips
  The target architecture is set to "mips".
  (gdb) show architecture
  The target architecture is set to "mips".
  (gdb)

The changes are:

  1. The value is now enclosed in quotes,
  2. Each line ends with '.', and
  3. After setting the architecture GDB is now a little more
  assertive; 'architecture is set to' not 'is assumed to be', the user
  did just tell us after all!

gdb/ChangeLog:

	* arch-utils.c (show_architecture): Update formatting of messages.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-osabi.exp: Update.
	* gdb.arch/arm-disassembler-options.exp: Update.
	* gdb.arch/powerpc-disassembler-options.exp: Update.
	* gdb.arch/ppc64-symtab-cordic.exp: Update.
	* gdb.arch/s390-disassembler-options.exp: Update.
	* gdb.base/all-architectures.exp.tcl: Update.
	* gdb.base/attach-pie-noexec.exp: Update.
	* gdb.base/catch-syscall.exp: Update.
	* gdb.xml/tdesc-arch.exp: Update.