sh: Move some code from onint() to onsig(), making onint() noreturn.

As a result, the INTON macro which is used many times generates fewer
bytes of code.
Allow -march=armv7a on the gcc command line, for compatibility with clang.
This will result in __ARM_ARCH_7A__ being defined during the compile.

When compiling with gcc, it will still only generate armv6 opcodes itself,
but should pass the arch to gas so that inline asm can use v7 opcodes.
Add __FreeBSD_version guards around hsearch_r to ease MFCing the code to
stable/10

It was added when __FreeBSD_version was ~1100027
Add -march=armv7a to the kernel compile for all ARM systems which are v7a.

Submitted by:	Michal Meloun <meloun@miracle.cz>
Replace the clock divisor terms with values that also result in a 1 MHz
clock, but actually work on real hardware, unlike the original set of
values I chose.

PR:		195009
Submitted by:	Scott Ellis <jumpnowtek@gmail.com>
Build selective portions of gnu/usr.bin/texinfo as part of build-tools to
ensure that building on a host without makeinfo (i.e. a host where
make delete-old -DWITHOUT_INFO was run), then building with MK_INFO == yes
doesn't manifest in build errors when building info pages

This manifested itself like the following when I was build testing an MFC
change on stable/10:

  makeinfo --no-split -I /usr/src/gnu/lib/libregex/doc -I /usr/src/gnu/lib/libregex/doc regex.texi  -o regex.info
  makeinfo: not found
  *** [regex.info] Error code 127

make[6]: stopped in /usr/src/gnu/lib/libregex/doc
1 error

Tested on a head VM without makeinfo installed and by building with MK_INFO=yes

MFC after: 1 week
Update the text for building texinfo with build-tools to reflect the fact
that make install is being called as well

MFC after: 1 week
X-MFC with: r276052