[arm] Allow explicit periods to be escaped in

The MULTILIB_REUSE mapping rules are built up using periods to
represent the placement of '=' signs in the command line syntax.  This
presents a problem if the option contains an explicit period because
that is translated unconditionally.  The result is that it is not
currently possible to write a reuse rule that would match the
ARMv8-M mainline architecture:

	-march=armv8-m.main

To fix this, this patch allows an explicit period to be escaped by writing
\. and by then preserving the period into the generated multilib header.

	* genmultilib (multilib_reuse): Allow an explicit period to be escaped
	with a backslash.  Remove the backslash after substituting unescaped
	periods.
	* doc/fragments.texi (MULTILIB_REUSE): Document it.

From-SVN: r249298