When this testcase was introduced it failed to account for the possibility of
targets that do not support arm mode or that do not support the ldrd/strd
instructions.

This patch accounts for both of these by adding some
dg-require-effective-target lines to the testcase.

This patch also adds a new effective-target procedure to check a target
supports ldrd/strd.

This patch also adds a new effective-target procedure to check a target
supports arm ldrd/strd.
The check uses the 'r' constraint to ensure SP is not used so that it will work
for thumb mode code generation as well as arm mode.

Tested by running this testcase with cross compilers using "-march=armv5t",
"-mcpu=cortex-m3", "-mcpu-arm7tdmi", "-mcpu=cortex-a9 -march=armv5t" for both
arm-none-eabi and arm-none-linux-gnueabihf.
Also ran this testcase with `make check` natively.

gcc/testsuite/ChangeLog:

2019-02-14  Matthew Malcomson  <matthew.malcomson@arm.com>

	* gcc.dg/rtl/arm/ldrd-peepholes.c: Restrict testcase.
	* lib/target-supports.exp: Add procedure to check for ldrd.

From-SVN: r268881