Save CSRs in interrupt functions

Link: https://inbox.sourceware.org/gcc-patches/20230613064126.1323-1-jinma@linux.alibaba.com/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: palmer-dabbelt

riscv-c-api.md

@@ -180,7 +180,9 @@ functions. Please consult your compiler's manual for more information.
 The interrupt attribute specifies that a function is an interrupt handler.
 The compiler will save/restore all used registers in the prologue/epilogue
 regardless of the ABI, all used registers including floating point
-register/vector register if `F` extension/vector extension is enabled.
+register/vector register if `F` extension/vector extension is enabled.  If F or
+V CSRs may be modified by an interrupt function, they must be saved by the
+compiler.
 
 The interrupt attribute can have an optional parameter to specify the mode.
 The possible values are `user`, `supervisor`, or `machine`.