std_detect: RISC-V Linux: Ergonomic querying with riscv_hwprobe

[a4lg]

Originally, we used an array of riscv_hwprobe directly and indexed using raw numbers, making correspondence between the index and the query key less obvious.

We also frequently used out[idx].key != -1 to test whether the key is supported by the riscv_hwprobe system call (on the Linux kernel version we are testing) but we'd better to integrate with an operation to retrieve the value.

This commit improves the ergonomics of feature querying by:

1. Utilizing macros to
   1. enable indexing by identifier names and
   2. encapsulate accesses to the riscv_hwprobe array to query and
2. New method riscv_hwprobe::get() returning Option<u64>, integrating availability checking and value retrieval.

It also removes has_ima for now because it's redundant if we only need to test for single base behavior.

Implementation

It (again) uses macro_rules! which is rare in architecture-specific portion of std_detect.

Due to hygiene requirements, outer init!{} macro generates inner data_mut!() and query![] macros that access inner variable. It makes sure that the inner variable t is only accessible through inner macros (data_mut!() provides effectively raw access but still controlled).

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