(Likely reason being that I don't understand what's going on, but would still appreciate if someone who is knowledgeable enough, would do the explaining.)

The clock_configure() function can be used to set up any of the clock. In that function there's a pair of hw_{clear,set}_bits() calls that write CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS to the clock's CTRL register.

But the value of CLOCKS_CLK_GPOUT0_CTRL_ENABLE_BITS, 0x00000800, does not make sense with clk_sys for example, because in that clock's CTRL register the bits 31:8 are marked as "Reserved" (2.15.7. List of Registers).