riscv: lib: uaccess: fold fixups into body

uaccess functions such __asm_copy_to_user(),  __arch_copy_from_user()
and __clear_user() place their exception fixups in the `.fixup` section
without any clear association with themselves. If we backtrace the
fixup code, it will be symbolized as an offset from the nearest prior
symbol.

Similar as arm64 does, we must move fixups into the body of the
functions themselves, after the usual fast-path returns.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>