add visibility=default to exported functions, visibility=hidden to most assembler #2285

Job	Run time
TEST (RISCV64_ZVL256B, TARGET=RISCV64_ZVL256B BINARY=64 ARCH=riscv64 BUILD_BFLOAT16=1 BUILD_HFLOA...	9m 8s
TEST (RISCV64_ZVL128B, TARGET=RISCV64_ZVL128B BINARY=64 ARCH=riscv64, rv64,g=true,c=true,v=true,v...	7m 35s
TEST (DYNAMIC_ARCH=1, TARGET=RISCV64_GENERIC BINARY=64 ARCH=riscv64 DYNAMIC_ARCH=1, rv64,g=true,c...	7m 28s
	24m 11s

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