Set architecture to arm in arm-*.xml files

Set architecture to arm in arm-*.xml files

This patch is to add the following line to various arm target description
xml files,

<architecture>arm</architecture>

in order to fix problems I've seen on aarch64 multi-arch debugging,

 detach^M
 Detaching from program: build-gdb/gdb/testsuite/gdb.base/attach, process 17145^M
 (gdb) PASS: gdb.base/attach.exp: attach1 detach file^M
 No executable file now.^M
 Architecture of file not recognized.^M
 (gdb) FAIL: gdb.base/attach.exp: attach1, purging symbols after detach

Without this patch, struct target_desc *tdesc_* are not initialised
properly, that is, fields arch and osabi in 'struct target_desc' are
not set properly.  This doesn't cause any problems on single arch
debugging, because arch-utils.c:gdbarch_info_fill will guess correctly.
However, in multi-arch debugging, gdbarch_info_fill gets the aarch64
arch, but the target description is for arm (because the current
inferior is 32-bit arm).

It is a surprise to me we didn't set architecture to "arm" before in *.xml
files, and I didn't find out why didn't do so.  AFAICS,
gdb/features/arm-with-iwmmxt.xml was added firstly (in patch
https://sourceware.org/ml/gdb-patches/2007-01/msg00593.html)
which had <architecture>iwmmxt</architecture>, however, afterwards,
architecture isn't set anymore in features/arm-*.xml files (in patches
https://sourceware.org/ml/gdb-patches/2009-07/msg00689.html and
https://sourceware.org/ml/gdb-patches/2010-08/msg00225.html).

gdb:

2015-07-07  Yao Qi  <yao.qi@linaro.org>

	* features/arm-with-m-fpa-layout.xml: Set architecture to arm.
	* features/arm-with-m-fpa-layout.c: Regenerated.
	* features/arm-with-m-vfp-d16.xml: Likewise.
	* features/arm-with-m-vfp-d16.c: Regenerated.
	* features/arm-with-m.xml: Likewise.
	* features/arm-with-m.c: Regenerated.
	* features/arm-with-neon.xml: Likewise.
	* features/arm-with-neon.c: Regenerated.
	* features/arm-with-vfpv2.xml: Likewise.
	* features/arm-with-vfpv2.c: Regenerated.
	* features/arm-with-vfpv3.xml: Likewise.
	* features/arm-with-vfpv3.c: Regenerated.