ARM interrupt attribute warning incorrect for M-profile

[john-brawn-arm]

Bugzilla Link	48471
Version	trunk
OS	Windows NT
CC	@zygoloid

Extended Description

If you compile the following

extern void fn(void);
attribute((interrupt)) void example(void) {
fn();
return;
}

with --target=arm-none-eabi -mcpu=cortex-m7 you get

tmp.c:3:3: warning: call to function without interrupt attribute could clobber interruptee's VFP registers [-Wextra]
fn();
^

This warning is incorrect for M-profile CPUs, as VFP registers are automatically saved and later restored in M-profile CPUs when entering an exception (or when a VFP register is modified in an exception handler if lazy context save is enabled).