Add the Armv8-M targets

This commit adds the Armv8-M targets recently added into Rust:
* Armv8-M Baseline ("thumbv8m.base-none-eabi")
* Armv8-M Mainline ("thumbv8m.main-none-eabi")
* Armv8-M Mainline with FPU extension ("thumbv8m.main-none-eabihf")

The FPU chosen for the hardfloat variant of Armv8-M Mainline is the one
available in the Cortex-M33 processor: single precision, FPv5
architecture.
It is described in the Arm Cortex-M33 Processor Technical Reference
Manual on the page 64.

src/lib.rs

@@ -1304,6 +1304,16 @@ impl Build {
                 if target.starts_with("thumbv7m") {
                     cmd.args.push("-march=armv7-m".into());
                 }
+                if target.starts_with("thumbv8m.base") {
+                    cmd.args.push("-march=armv8-m.base".into());
+                }
+                if target.starts_with("thumbv8m.main") {
+                    cmd.args.push("-march=armv8-m.main".into());
+
+                    if target.ends_with("eabihf") {
+                        cmd.args.push("-mfpu=fpv5-sp-d16".into())
+                    }
+                }
                 if target.starts_with("armebv7r") | target.starts_with("armv7r") {
                     if target.starts_with("armeb") {
                         cmd.args.push("-mbig-endian".into());
@@ -1758,6 +1768,9 @@ impl Build {
                         "thumbv7em-none-eabi" => Some("arm-none-eabi"),
                         "thumbv7em-none-eabihf" => Some("arm-none-eabi"),
                         "thumbv7m-none-eabi" => Some("arm-none-eabi"),
+                        "thumbv8m.base-none-eabi" => Some("arm-none-eabi"),
+                        "thumbv8m.main-none-eabi" => Some("arm-none-eabi"),
+                        "thumbv8m.main-none-eabihf" => Some("arm-none-eabi"),
                         "x86_64-pc-windows-gnu" => Some("x86_64-w64-mingw32"),
                         "x86_64-rumprun-netbsd" => Some("x86_64-rumprun-netbsd"),
                         "x86_64-unknown-linux-musl" => Some("musl"),