add support for gmp 6.1.1 and upgrade default version

this fixes build failures for native compiler on some targets where
linking gmp fails with undefined references to __gmpn_invert_limb.

Makefile

@@ -6,7 +6,7 @@ CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.25.1
 GCC_VER = 5.3.0
 MUSL_VER = 1.1.15
-GMP_VER = 6.1.0
+GMP_VER = 6.1.1
 MPC_VER = 1.0.3
 MPFR_VER = 3.1.4
 LINUX_VER = 4.4.10

hashes/gmp-6.1.1.tar.bz2.sha1

@@ -0,0 +1 @@
+757d672e66d8e0afe60ca04735ab11c00d9346e4  gmp-6.1.1.tar.bz2