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_span/src/symbol.rs

@@ -218,6 +218,7 @@ symbols! {
         abi,
         abi_amdgpu_kernel,
         abi_avr_interrupt,
+        abi_c_cmse_nonsecure_call,
         abi_efiapi,
         abi_msp430_interrupt,
         abi_ptx,