kconfig: Rename CONFIG_FP_SHARING to CONFIG_FPU_SHARING

This commit renames the Kconfig `FP_SHARING` symbol to `FPU_SHARING`,
since this symbol specifically refers to the hardware FPU sharing
support by means of FPU context preservation, and the "FP" prefix is
not fully descriptive of that; leaving room for ambiguity.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>