Arm: Allow version strings in the triplet regexp

Arm: Allow version strings in the triplet regexp

On Arm, the OS may use the full version string for the arch name when
installing the compiler, for example armv7hl-redhat-linux-gnueabi-gcc.

Implement gdbarch_gnu_triplet_regexp for Arm to allow this to be detected.
Ensure that other Arm targets (eg iwmmxt) are not affected.

This fixes the compile/ set of tests on those systems.

gdb/ChangeLog:

2019-06-26  Alan Hayward  <alan.hayward@arm.com>

	* arm-tdep.c (arm_gnu_triplet_regexp): New function.
	(arm_gdbarch_init): Add arm_gnu_triplet_regexp.