testsuite: Add -fwrapv to signbit-5.c

On Cortex-M55 with MVE, the test case fails due to -INT_MAX being
undefined. Adding -fwrapv solves the issues.

Regtested on x86_64-pc-linux-gnu and arm-none-eabi for
Cortex-M0/M3/M4/M7/M33/M55/M85/A7.

gcc/testsuite/ChangeLog:

	* gcc.dg/signbit-5.c: Add -fwrapv and remove x86 exception.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Co-authored-by: Yvan ROUX <yvan.roux@foss.st.com>