[PR51447] restore the global reg var before returning from main

[PR51447] restore the global reg var before returning from main

A runtime system might legitimately hold in rbx a value expected to be
preserved across the call to main, but its use as a global register
variable stops main from preserving it.

for  gcc/testsuite/ChangeLog

	PR rtl-optimization/51447
	* gcc.c-torture/execute/pr51447.c (main): Preserve call-saved
	register.