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cmd_record_mcount is used to locate the _mcount symbols in the object
files, only the files compiled with -pg has the _mcount symbol, so, it
is only needed for such files, but the current cmd_record_mcount is used
for all of the object files, so, we need to fix it and speed it up.

Since -pg may be removed by the method used in kernel/trace/Makefile:

ORIG_CFLAGS := $(KBUILD_CFLAGS)
KBUILD_CFLAGS = $(subst -pg,,$(ORIG_CFLAGS))

Or may be removed by the method used in arch/x86/kernel/Makefile:

CFLAGS_REMOVE_file.o = -pg

So, we must check the last variable stores the compiling flags, that is
c_flags(Please refer to cmd_cc_o_c and rule_cc_o_c defined in
scripts/Makefile.build) and since the CFLAGS_REMOVE_file.o is already
filtered in _c_flags(Please refer to scripts/Makefile.lib) and _c_flags
has less symbols, therefore, we only need to check _c_flags.

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Changes from v1:

  o Don't touch Makefile for CONFIG_FTRACE_MCOUNT_RECORD is enough
  o Use _c_flags intead ...