um: Increase stack frame size threshold for signal.c

The signal.c can't use heap for bit data located on stack. However,
by default a compiler warns us about overstepping stack frame size
threshold:

arch/um/os-Linux/signal.c: In function ‘sig_handler_common’:
arch/um/os-Linux/signal.c:51:1: warning: the frame size of 2960 bytes is larger than 2048 bytes [-Wframe-larger-than=]
   51 | }
      | ^
arch/um/os-Linux/signal.c: In function ‘timer_real_alarm_handler’:
arch/um/os-Linux/signal.c:95:1: warning: the frame size of 2960 bytes is larger than 2048 bytes [-Wframe-larger-than=]
    95 | }
       | ^

Due to above increase stack frame size threshold explicitly for signal.c
to avoid unnecessary warning.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/Makefile

@@ -10,6 +10,8 @@ obj-y = execvp.o file.o helper.o irq.o main.o mem.o process.o \
 	registers.o sigio.o signal.o start_up.o time.o tty.o \
 	umid.o user_syms.o util.o drivers/ skas/
 
+CFLAGS_signal.o += -Wframe-larger-than=4096
+
 obj-$(CONFIG_ARCH_REUSE_HOST_VSYSCALL_AREA) += elf_aux.o
 
 USER_OBJS := $(user-objs-y) elf_aux.o execvp.o file.o helper.o irq.o \