Mark internal gmp functions with attribute_hidden [BZ #18822]

Mark internal gmp functions with attribute_hidden [BZ #18822]

Mark internal gmp functions with attribute_hidden to allow direct
access within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/gmp.h: Declare internal functions only if _ISOMAC is
	undefined.
	(__mpn_extract_double): Add attribute_hidden.
	(__mpn_extract_long_double): Likewise.
	(__mpn_extract_float128): Likewise.
	(__mpn_construct_float): Likewise.
	(__mpn_construct_double): Likewise.
	(__mpn_construct_long_double): Likewise.
	(__mpn_construct_float128): Likewise.
	(mpn_add_1): Likewise.
	(mpn_addmul_1): Likewise.
	(mpn_add_n): Likewise.
	(mpn_cmp): Likewise.
	(mpn_divrem): Likewise.
	(mpn_lshift): Likewise.
	(mpn_mul): Likewise.
	(mpn_mul_1): Likewise.
	(mpn_rshift): Likewise.
	(mpn_sub_1): Likewise.
	(mpn_submul_1): Likewise.
	(mpn_sub_n): Likewise.