hadrian: fix CFLAGS for gmp shared objs on wasm

This commit adds -fvisibility=default to CFLAGS of gmp when building
for wasm. This is required to generate the ghc-bignum shared library
without linking errors. Clang defaults to -fvisibility=hidden for wasm
targets, which will cause issues when a symbol is expected to be
exported in a shared library but without explicit visibility attribute
annotation.