RISC-V: Define indirect CSR access helpers

The indirect CSR requires multiple instructions to read/write CSR.
Add a few helper macros for ease of usage.

These have to be macros rather than functions. csr_read()/csr_write()
stringify their CSR argument into the inline asm template via
__ASM_STR(), so the CSR number must be a literal token; passing it as a
function parameter emits "csrr %0, iregcsr", which the assembler rejects
with "unknown CSR `iregcsr'". The stringification happens in the
preprocessor, before inlining or constant propagation, so it cannot be
worked around by forcing inlining or by only ever passing constants -
gcc 12, gcc 16 and clang 22 all reject it alike. Underneath, csrr/csrw
encode the CSR as a 12-bit immediate and RISC-V has no register-indirect
form, which is also why asm/csr.h keeps every one of its accessors as a
macro.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
[pjw@kernel.org: expand "ind" abbreviation]
Link: https://patch.msgid.link/20260807-counter_delegation-v9-5-58658104e487@meta.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

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