gmp_5.0.2: Set CC_FOR_BUILD to BUILD_CC

gmp_5.0.2: Set CC_FOR_BUILD to BUILD_CC

CC_FOR_BUILD was compiling the test programs using the target's
compile options and executing those on the host, causing errors such
as:

/bin/sh: line 1: 15032 Illegal instruction     ./gen-bases table 64 0 > mpn/mp_bases.c
/bin/sh: line 1: 15033 Illegal instruction     ./gen-bases header 64 0 > mp_bases.h

Export CC_FOR_BUILD using BUILD_CC to fix the problem.

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>