atomic-arm.h: implement armv6 support

Change-Id: Ifdbaa0b8296e8df966dc2259b0a753f477c5ec69
Signed-off-by: Marco Milanese <marcomilanese7@gmail.com>
debuggerd: Extend condition for exclusion of the fconstd instruction

The fconstd instruction only exists in ARM cores with VFPv3 or
later. Currently all supported ARMv6 targets do not have VFPv3 or
later, so let's just extend based on this trend.

Change-Id: I2ff9a7653eeadb93d5ed9e25a59a0109b1dc0c95