[arm] PR target/85606 prefer armv6s-m for armv6-m parts

[arm] PR target/85606 prefer armv6s-m for armv6-m parts

When Arm introduced ARMv6-M there were two variants, ARMv6-M and
ARMv6S-M.  The two differed only in support for the SVC instruction.
Later on SVC was then made a mandatory part of ARMv6-M and the
ARMv6S-M name was dropped.  GCC and GAS, however still recognize both
names and at least some versions of GAS still distinguish between the
two.

To address this, this patch changes the architecture for the ARMv6-m
cortex parts (m0, m0plus, m1 and the variants will small multiply
units) to use the ARMv6S-M name in conjunction with the assembler.
This avoids problems with them rejecting code that was previously
accepted with older versions of GCC where we did not pass an explicit
architecture string through to the compiler when using -mcpu on the
command line.

	PR target/85606
	* config/arm/arm-cpus.in: Add comment that ARMv6-m and ARMv6S-m are now
	equivalent.
	(cortex-m0): Use armv6s-m isa.
	(cortex-m0plus): Likewise.
	(cortex-m1): Likewise.
	(cortex-m0.small-multiply): Likewise.
	(cortex-m0plus.small-multiply): Likewise.
	(cortex-m1.small-multiply): Likewise.

From-SVN: r260157