Remove User Interrupts

These have been removed from both GCC and Clang, because the underlying
N extension they were based on was never ratified.

This change also relaxes some of the requirements, to allow vendor
interrupt modes, and combinations of (compatible) interrupt modes, which
matches the LLVM implementation of SiFive interrupts.

Author: lenary

src/c-api.adoc

@@ -183,7 +183,7 @@ This attribute is incompatible with the `interrupt` attribute.
 NOTE: Be aware that compilers might have further restrictions on naked
 functions. Please consult your compiler's manual for more information.
 
-=== `+__attribute__((interrupt))+`, `+__attribute__((interrupt("user")))+`, `+__attribute__((interrupt("supervisor")))+`, `+__attribute__((interrupt("machine")))+`
+=== `+__attribute__((interrupt))+`, `+__attribute__((interrupt("supervisor")))+`, `+__attribute__((interrupt("machine")))+`
 
 The interrupt attribute specifies that a function is an interrupt handler.
 The compiler will save/restore all used registers in the prologue/epilogue
@@ -192,12 +192,13 @@ 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`.
+The interrupt attribute can have optional parameters to specify the mode.
+The possible values are `supervisor`, or `machine`, or vendor-specific values.
 The default value `machine` is used, if the mode is not specified.
 
-The function can specify only one mode; the compiler should raise an error if a
-function declares more than one mode or an undefined mode.
+The compiler should raise an error if a function declares incompatible modes, or
+an undefined mode. The `supervisor` mode is incompatible with the `machine`
+mode.
 
 This attribute is incompatible with the `naked` attribute.