riscv-rt: rename no-interrupts and no-exceptions to custom-interrupts and custom-exceptions

[romancardenas]

As discussed in #390 , custom-* is a better naming than no-*, given that no-interrupts means that a target has a custom set of interrupts (it does not mean that there is no interrupts in the target).

This PR renames no-interrupts to custom-interrupts and no-exceptions to custom-exceptions.

For backwards compatibility, the old features remain and work as expected. However, they now trigger a warning message encouraging users to adopt the new naming:

cargo build -p riscv-rt --features no-interrupts,no-exceptions

   Compiling riscv-rt v0.19.0 (/Users/rcardenas/embedded-rust/riscv/riscv-rt)
warning: use of deprecated constant `__RISCV_RT_DEPRECATED_NO_INTERRUPTS`: feature `no-interrupts` is deprecated; use `custom-interrupts` instead
   --> riscv-rt/src/lib.rs:695:13
    |
695 |     let _ = __RISCV_RT_DEPRECATED_NO_INTERRUPTS;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

warning: use of deprecated constant `__RISCV_RT_DEPRECATED_NO_EXCEPTIONS`: feature `no-exceptions` is deprecated; use `custom-exceptions` instead
   --> riscv-rt/src/lib.rs:707:13
    |
707 |     let _ = __RISCV_RT_DEPRECATED_NO_EXCEPTIONS;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `riscv-rt` (lib) generated 2 warnings