Add a new ABI to support cmse_nonsecure_call

This commit adds a new ABI to be selected via `extern
"C-cmse-nonsecure-call"` on function pointers in order for the compiler to
apply the corresponding cmse_nonsecure_call callsite attribute.
For Armv8-M targets supporting TrustZone-M, this will perform a
non-secure function call by saving, clearing and calling a non-secure
function pointer using the BLXNS instruction.

See the page on the unstable book for details.

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

compiler/rustc_target/src/abi/call/mod.rs

@@ -551,6 +551,7 @@ pub enum Conv {
 
     // Target-specific calling conventions.
     ArmAapcs,
+    CCmseNonSecureCall,
 
     Msp430Intr,
 

compiler/rustc_target/src/spec/abi.rs

@@ -36,6 +36,7 @@ pub enum Abi {
     EfiApi,
     AvrInterrupt,
     AvrNonBlockingInterrupt,
+    CCmseNonSecureCall,
 
     // Multiplatform / generic ABIs
     System,
@@ -81,6 +82,7 @@ const AbiDatas: &[AbiData] = &[
         name: "avr-non-blocking-interrupt",
         generic: false,
     },
+    AbiData { abi: Abi::CCmseNonSecureCall, name: "C-cmse-nonsecure-call", generic: false },
     // Cross-platform ABIs
     AbiData { abi: Abi::System, name: "system", generic: true },
     AbiData { abi: Abi::RustIntrinsic, name: "rust-intrinsic", generic: true },