gcov: Add __gcov_info_to_gdca()

Add __gcov_info_to_gcda() to libgcov to get the gcda data for a gcda info in a
freestanding environment.  It is intended to be used with the
-fprofile-info-section option.  A crude test program which doesn't use a linker
script is (use "gcc -coverage -fprofile-info-section -lgcov test.c" to compile
it):

  #include <gcov.h>
  #include <stdio.h>
  #include <stdlib.h>

  extern const struct gcov_info *my_info;

  static void
  filename (const char *f, void *arg)
  {
    printf("filename: %s\n", f);
  }

  static void
  dump (const void *d, unsigned n, void *arg)
  {
    const unsigned char *c = d;

    for (unsigned i = 0; i < n; ++i)
      printf ("%02x", c[i]);
  }

  static void *
  allocate (unsigned length, void *arg)
  {
    return malloc (length);
  }

  int main()
  {
    __asm__ volatile (".set my_info, .LPBX2");
    __gcov_info_to_gcda (my_info, filename, dump, allocate, NULL);
    return 0;
  }

With this patch, <stdint.h> is included in libgcov-driver.c even if
inhibit_libc is defined.  This header file should be also available for
freestanding environments.  If this is not the case, then we have to define
intptr_t somehow.

The patch removes one use of memset() which makes the <string.h> include
superfluous.

gcc/

	* gcov-io.h (gcov_write): Declare.
	* gcov-io.c (gcov_write): New.
	(gcov_write_counter): Remove.
	(gcov_write_tag_length): Likewise.
	(gcov_write_summary): Replace gcov_write_tag_length() with calls to
	gcov_write_unsigned().
	* doc/invoke.texi (fprofile-info-section): Mention
	__gcov_info_to_gdca().

gcc/testsuite/

	* gcc.dg/gcov-info-to-gcda.c: New test.

libgcc/

	* Makefile.in (LIBGCOV_DRIVER): Add _gcov_info_to_gcda.
	* gcov.h (gcov_info): Declare.
	(__gcov_info_to_gdca): Likewise.
	* libgcov.h (gcov_write_counter): Remove.
	(gcov_write_tag_length): Likewise.
	* libgcov-driver.c (#include <stdint.h>): New.
	(#include <string.h>): Remove.
	(NEED_L_GCOV): Conditionally define.
	(NEED_L_GCOV_INFO_TO_GCDA): Likewise.
	(are_all_counters_zero): New.
	(gcov_dump_handler): Likewise.
	(gcov_allocate_handler): Likewise.
	(dump_unsigned): Likewise.
	(dump_counter): Likewise.
	(write_topn_counters): Add dump_fn, allocate_fn, and arg parameters.
	Use dump_unsigned() and dump_counter().
	(write_one_data): Add dump_fn, allocate_fn, and arg parameters.  Use
	dump_unsigned(), dump_counter(), and are_all_counters_zero().
	(__gcov_info_to_gcda): New.