libgcov: support overloaded malloc

gcc/ChangeLog:

	* gcov-io.h (GCOV_PREALLOCATED_KVP): New.

libgcc/ChangeLog:

	* libgcov-driver.c: Add __gcov_kvp_pool
	and __gcov_kvp_pool_index variables.
	* libgcov.h (allocate_gcov_kvp): New.
	(gcov_topn_add_value): Use it.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-prof/indir-call-prof-malloc.c: New test.