tools: testing: support EXTRA_CFLAGS in shared.mk

This allows the user to set cflags when building tests that use this
shared build infrastructure.

For example, it enables building with -Werror so that patch-check scripts
will fail:

make -C tools/testing/vma -j EXTRA_CFLAGS=-Werror

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Brendan Jackman <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Reviewed-by: Liam R. Howlett <[e-mail]>
Acked-by: Pedro Falcato <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/shared/shared.mk

···
 CFLAGS += -I../shared -I. -I../../include -I../../arch/$(SRCARCH)/include \
 	  -I../../../lib -g -Og -Wall \
 	  -D_LGPL_SOURCE -fsanitize=address -fsanitize=undefined
+CFLAGS += $(EXTRA_CFLAGS)
 LDFLAGS += -fsanitize=address -fsanitize=undefined
 LDLIBS += -lpthread -lurcu
 LIBS := slab.o find_bit.o bitmap.o hweight.o vsprintf.o