testsuite: arm: Use -mtune=cortex-m4 for thumb-ifcvt.c test

testsuite: arm: Use -mtune=cortex-m4 for thumb-ifcvt.c test

On Cortex-M4, the code generated is:
     cmp     r0, r1
     itte    ne
     lslne   r0, r0, r1
     asrne   r0, r0, #1
     moveq   r0, r1
     add     r0, r0, r1
     bx      lr

On Cortex-M7, the code generated is:
     cmp     r0, r1
     beq     .L3
     lsls    r0, r0, r1
     asrs    r0, r0, #1
     add     r0, r0, r1
     bx      lr
.L3:
     mov     r0, r1
     add     r0, r0, r1
     bx      lr

As Cortex-M7 only allow maximum one conditional instruction, force
Cortex-M4 to have a stable test case.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/thumb-ifcvt.c: Use -mtune=cortex-m4.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>