feat(arm): add legacy Armv4 and Armv4T support

- Added Armv4 and Armv4T variants to Arch enum with proper doc comments
- Updated Arch::get, Arch::values, and Display to include new variants
- Updated Arch::profile mapping to include Armv4/Armv4T
- Updated Isa::get to assign correct instruction set to legacy architectures
- Ensured TargetInfo populates arch, isa, and profile for all supported targets

Author: KushalMeghani1644

arm-targets/src/lib.rs

@@ -77,6 +77,7 @@ impl Isa {
     pub fn get(target: &str) -> Option<Isa> {
         let arch = Arch::get(target)?;
         Some(match arch {
+            Arch::Armv4 | Arch::Armv4T => Isa::A32,
             Arch::Armv6M => Isa::T32,
             Arch::Armv7M => Isa::T32,
             Arch::Armv7EM => Isa::T32,
@@ -118,6 +119,10 @@ impl core::fmt::Display for Isa {
 /// As defined by a particular revision of the Arm Architecture Reference Manual (ARM).
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum Arch {
+    /// Armv4 (legacy, also known as ARMv4)
+    Armv4,
+    /// Armv4T (legacy, also known as ARMv4T)
+    Armv4T,
     /// Armv6-M (also known as ARMv6-M)
     Armv6M,
     /// Armv7-M (also known as ARMv7-M)
@@ -141,7 +146,11 @@ pub enum Arch {
 impl Arch {
     /// Decode a target string
     pub fn get(target: &str) -> Option<Arch> {
-        if target.starts_with("thumbv6m-") {
+        if target.starts_with("armv4-") {
+            Some(Arch::Armv4)
+        } else if target.starts_with("armv4t-") {
+            Some(Arch::Armv4T)
+        } else if target.starts_with("thumbv6m-") {
             Some(Arch::Armv6M)
         } else if target.starts_with("thumbv7m-") {
             Some(Arch::Armv7M)
@@ -170,14 +179,16 @@ impl Arch {
             Arch::Armv6M | Arch::Armv7M | Arch::Armv7EM | Arch::Armv8MBase | Arch::Armv8MMain => {
                 Profile::M
             }
-            Arch::Armv7R | Arch::Armv8R => Profile::R,
+            Arch::Armv4 | Arch::Armv4T | Arch::Armv7R | Arch::Armv8R => Profile::R,
             Arch::Armv7A | Arch::Armv8A => Profile::A,
         }
     }
 
     /// Get a comma-separated list of values, suitable for cfg-check
     pub fn values() -> String {
         let string_versions: Vec<String> = [
+            Arch::Armv4,
+            Arch::Armv4T,
             Arch::Armv6M,
             Arch::Armv7M,
             Arch::Armv7EM,
@@ -201,6 +212,8 @@ impl core::fmt::Display for Arch {
             f,
             "{}",
             match self {
+                Arch::Armv4 => "v4",
+                Arch::Armv4T => "v4t",
                 Arch::Armv6M => "v6-m",
                 Arch::Armv7M => "v7-m",
                 Arch::Armv7EM => "v7e-m",